2 . Control

Chapter Chair

Grahame Grieve Kestral Computing Pty Ltd.

Chapter Chair and Editor: (2)

Anthony Julian Mayo Clinic

Chapter Chair

Scott Robertson Kaiser Permanente

Chapter Chair and Editor (2A)

Doug Pratt Siemens Medical Solutions Health Services

Chapter Chair

René Spronk Ringholm GmbH

Implementation/Conformance TC Co-Chair

Lisa Carnahan National Institute of Standards and Technology (NIST)

Implementation/Conformance TC Co-Chair

Frank Oemig Agfa HealthCare GmbH

Implementation/Conformance TC Co-Chair

Charlie McCay Ramsey Systems

Implementation/Conformance TC Co-Chair

John Lyons Siemens Medical Solutions Health Services

2.1 CHAPTER 2 CONTENTS

2.2 INTRODUCTION

The Control chapter of this Standard defines the generic rules that apply to all messages. Subsequent sections define functionally specific messages to be exchanged among certain applications. The specific aspects of message definition that are addressed herein are:

1. the form to be used in functional chapters for describing messages. This includes their purpose, their contents, and the interrelationships among them. This form is called an abstract message definition because it is purely a level 7 (application) definition.

2. the HL7 encoding rules for converting an abstract message into a string of characters that comprises an actual message.

3. the programming procedures required to exchange messages using the HL7 specifications.

4. the anticipated relationship with lower level protocols.

5. certain message segments that are components of all messages.

6. a single message, the acknowledgment message, that may be used unchanged in multiple applications.

2.3 CONCEPTUAL APPROACH

2.3.1 Trigger events

The Standard is written from the assumption that an event in the real world of healthcare creates the need for data to flow among systems. The real-world event is called the trigger event. For example, the trigger event a patient is admitted may cause the need for data about that patient to be sent to a number of other systems. The trigger event, an observation (e.g., a CBC result) for a patient is available, may cause the need for that observation to be sent to a number of other systems. When the transfer of information is initiated by the application system that deals with the triggering event, the transaction is termed an unsolicited update.

Note: No assumption is made about the design or architecture of the application system creating the unsolicited update. The scope of HL7 is restricted to the specification of messages between application systems and the events triggering them.

HL7 allows the use of trigger events at several different levels of data granularity and inter-relationships. For example, most Patient Administration (ADT) trigger events concern single objects (such as an admit event, which creates a message that contains data about a single person and/or account). Other ADT trigger events are concerned with relationships between more than one object (e.g., the merge events, which specify patient or account merges). Some ADT trigger events pertain to a collection of objects that may have no significant inter-relationships (e.g., a record-oriented location-based query, whose response contains data about a collection of inpatients who are related only temporarily, by local geography).

2.3.2 Acknowledgments: original mode

When the unsolicited update is sent from one system to another, this acknowledgment mode specifies that it be acknowledged at the application level. The reasoning is that it is not sufficient to know that the underlying communications system guaranteed delivery of the message. It is also necessary to know that the receiving application processed the data successfully at a logical application level.

The acknowledgment may contain data of interest to the system that initiated the exchange. For example, if a patient care system has processed the trigger event a lab test is ordered for a patient, it may send an unsolicited update to a lab application identifying the patient, the test ordered, and various other information about the order. The ancillary system will acknowledge the order when it has processed it successfully. For some pairings of patient care and ancillary department systems the acknowledgment may also include the ancillary identification number that was assigned (HL7 does not require Order Entry and Results Reporting applications to interface in this manner, but it supports those that do).

The HL7 Standard makes no assumptions about the ownership of data. It also makes no requirements of its own on the subsequent action of the recipient of data, nor does it make any assumption about the design or architecture of the receiving application system. The scope of HL7 is restricted to the specification of messages between application systems, and the events triggering them. HL7 does not explicitly support, but can be used with, systems that support store and forward and data broadcast facilities (see the HL7 Implementation Support Guide).

The HL7 Standard makes no functional interpretation of the requirement that a system commit the data in a message to its database before acknowledging it. All that is required is that the receiving system accept responsibility for the data, providing the same integrity test that it would apply to data from any source. To continue the prior example, the ancillary system may acknowledge the order after placing it in an input queue, expecting to fully process the order into its database at a future time. The only assumption is that the input queue is maintained at the same level of integrity as the database.

Instances of messages are transient by nature, and can not be expected by transmitter and/or receiver to be persistent after acknowledgement.

2.3.3 Acknowledgments: enhanced mode

The HL7 acknowledgment paradigm has been extended to distinguish both accept and application acknowledgments, as well the conditions under which each is required. With a positive accept acknowledgment, the receiving system commits the message to safe storage in a manner that releases the sending system from the need to resend the message. After the message has been processed by the receiving system, an application acknowledgment may be used to return the resultant status to the sending system.

2.3.4 Queries

Query documentation including messages, segments, special protocols, implementation considerations and examples have been moved to chapter 5. The unsolicited display messages were also moved because their message syntax is query-like in nature.

2.4 COMMUNICATIONS ENVIRONMENT

The HL7 Standard defines the messages as they are exchanged among application entities and the procedures used to exchange them. As such, it conceptually operates at the seventh level of the ISO model for Open System Interconnection (OSI). It is primarily concerned with the data content and interrelationship of messages and with communicating certain application-level error conditions.

Since the OSI protocols are not universally implemented, the HL7 Working Group is interested in providing standards that will be useful in the interim. It is also recognized that there is now, and will continue to be, interest in communicating health data among systems operating in communications environments that provide a high level of functionality, but use protocols other than ISO OSI. The universe of environments of interest to HL7 includes, but is not restricted to:

1. ad hoc environments that do not provide even basic transport reliability. Such environments consist of point-to-point RS-232 links, modems, and even LANs, if their connection to host computers is made via RS-232 communications links. Until OSI high level standards become truly prevalent, many healthcare interfaces will be implemented over such links. In such an environment, the HL7 Lower Level Protocols (LLP) may be used between systems to enhance the capabilities of the communications environment. The HL7 Lower Level Protocols are defined in the HL7 Implementation Guide, which is not an official part of the Standard.

2. environments that support a robust transport level, but do not meet the high level requirements. This includes environments such as TCP/IP, DECNET, and SNA.

3. ISO and proprietary networks that implement up to presentation and other high level services. IBM's SNA LU6.2 and SUN Microsystems's NFS are examples of complete proprietary networks.

4. two or more applications running on the same physical and/or logical machine that are not tightly integrated. In these environments, the messaging capabilities may be provided by inter-process communications services (e.g., Pipes in a UNIX System).

The HL7 Standard assumes that the communications environment will provide the following capabilities:

1. error free transmission. Applications can assume that they correctly received all of the transmitted bytes in the order in which they were sent. This implies that error checking is done at a lower level. However, sending applications may not assume that the message was actually received without receiving an acknowledgment message.

2. character conversion. If the two machines exchanging data use different representations of the same character set, the communications environment will convert the data from one representation to the other.

3. message length. HL7 sets no limits on the maximum size of HL7 messages. The Standard assumes that the communications environment can transport messages of any length that might be necessary. In practice, sites may agree to place some upper bound on the size of messages and may use the message continuation protocol, described later in this chapter, for messages that exceed the upper limit.

Note: Just as HL7 makes no assumptions about the design or architecture of the application systems sending and receiving HL7 messages, it makes no assumptions about the communications environment beyond those listed above. In particular, aside from the above assumptions, the communications environment, including its architecture, design and implementation, is outside the scope of HL7.

2.5 MESSAGE FRAMEWORK

This section defines the constituents of messages and provides the methodology for defining abstract messages that are used in later chapters. Message construction rules can be found in section 2.6.

2.5.1 Messages

A message is the atomic unit of data transferred between systems. It is comprised of a group of segments in a defined sequence. Each message has a message type that defines its purpose. For example the ADT Message type is used to transmit portions of a patient's Patient Administration (ADT) data from one system to another. A three-character code contained within each message identifies its type. These are listed in the Message Type list, Appendix A.

The real-world event that initiates an exchange of messages is called a trigger event. See Section 2.3.1, "Trigger events," for a more detailed description of trigger events. Refer to HL7 Table 0003 - Event type for a listing of all defined trigger events. These codes represent values such as A patient is admitted or An order event occurred. There is a one-to-many relationship between message types and trigger event codes. The same trigger event code may not be associated with more than one message type; however a message type may be associated with more than one trigger event code.

All message types and trigger event codes beginning with the letter "Z" are reserved for locally defined messages. No such codes will be defined within the HL7 Standard.

2.5.2 Segments and segment groups

A segment is a logical grouping of data fields. Segments of a message may be required or optional. They may occur only once in a message or they may be allowed to repeat. Each segment is given a name. For example, the ADT message may contain the following segments: Message Header (MSH), Event Type (EVN), Patient ID (PID), and Patient Visit (PV1).

Each segment is identified by a unique three-character code known as the Segment ID. Although the actual segments are defined in various chapters, the ID codes assigned to the various segments are listed in Appendix A.

All segment ID codes beginning with the letter Z are reserved for locally defined segments. No such codes will be defined within the HL7 Standard.

Two or more segments may be organized as a logical unit called a segment group. A segment group may be required or optional and might or might not repeat. As of v 2.5, the first segment in a newly defined segment group will be required to help ensure that unparsable messages will not be inadvertently defined. This required first segment is known as the anchor segment.

A segment group is assigned a name that represents a permanent identifier that may not be changed.

A named segment X may occur more than once in an abstract message syntax. This differs from repetition described earlier in this section. When this occurs, the following rules must be adhered to:

If, within an abstract message syntax, a named segment X appears in two individual or group locations, and

1. Either appearance is optional or repeating in an individual location;

2. or, either appearance is optional or repeating, in a group location

then, the occurrences of segment X must be separated by at least one required segment of a different name so that no ambiguity can exist as to the individual or group location of any occurrence of segment X in a message instance.

Examples of proper segment grouping

Examples of unparsable segment grouping

In each of these examples it is not possible to tell which part of the message SEG1 belongs.

2.5.3 Fields

Definition: A field is a string of characters. Fields for use within HL7 segments are defined by HL7. A comprehensive data dictionary of all HL7 fields is provided in Appendix A.

HL7 does not care how systems actually store data within an application. When fields are transmitted, they are sent as character strings

A field may exist in one of three population states in an HL7 message:

Populated. (Synonyms: valued, non-blank, not blank, not empty.) The sending system sends a value in the field. For example, if a sending system includes medical record number, that would be communicated as |1234567^^^MR^KP-CA|. Note that the field might be populated with a code that means "no information" or "unknown".

Not populated. (Synonyms: unpopulated, not valued, unvalued, blank, empty, not present, missing.) The sending system does not supply a value for the field. The Sender might or might not have a value for the field. The receiving system can make no inference regarding the absence of an element value if there is not a conformance profile governing the implementation. However, if there is a Conformance Message Profile in effect, then special rules apply; see section 2.B, "Conformance Using Message Profiles".

Null. Any existing value for the corresponding data base element in the receiving application should be deleted. This is symbolically communicated as two double-quotes between the delimiters (i.e., |""|).Employing consecutive double quote characters as the only content of a field for other purposes is prohibited.

Refer to Section 2.6, "Message Construction Rules" for information on data fields with a null value.

Refer to section 2.10.4, "Protocol for interpreting repeating segments or segment groups in an update Message" for information on updating records in a database.

Version control rules regarding fields can be found in section 2.8, "Version compatibility definition".

Local extension rules regarding fields can be found in section 2.11, "Local Extension".

The various chapters of the Standard contain segment attribute tables. These tables list and describe the data fields in the segment and characteristics of their usage. In defining a segment, the following information is specified about each field:

2.5.3.1 Position (sequence within the segment)

Definition: Ordinal position of the data field within the segment. This number is used to refer to the data field in the text comments that follow the segment definition table.

In the segment attribute tables this information is provided in the column labeled SEQ.

2.5.3.2 Maximum length

Definition: Maximum number of characters that one occurrence of the data field may occupy.

In the segment attribute tables this information is in a column labeled LEN.

The maximum length is not of conceptual importance in the abstract message or the HL7 coding rules. The length of a field is normative. Changes to the field length may be negotiated by a site agreement such as a conformance profile. See section 2.B, "Conformance Using Message Profiles". When this is done, it shall not render the implementation non-conformant. The receiver must be able to receive up to the maximum field length, and the sender can send up to the maximum field length.

Field length is determined based on the data type lengths, and should fall between the lower and the upper bounds for the corresponding data types. It is calculated to include the component and subcomponent separators. Because the maximum length is that of a single occurrence, the repetition separator is not included in calculating the maximum length See Section 2.5.3.5, "Repetition".

The following conventions have been applied:

1. The maximum length of the data field shall be expressed as a number.

2. If the maximum length needs to convey the notion of a Very Large Number, the number 65536 should be displayed to alert the user. This convention takes the place of the practice in versions prior to 2.4 of abbreviating this expression as 64K.

3. If the maximum length cannot be definitively expressed because the data type for the field is variable, the symbolic number 99999 should be displayed. This convention takes the place of the practice in versions prior to 2.4 of displaying the notation "varies" or some other non-numeric description.

In v 2.5 maximum lengths are being assigned to data types. See HL7 Table 0440 - Data Types.

2.5.3.3 Data type

Definition: The basic building block used to construct or restrict the contents of a data field.

In the segment attribute tables this information is provided in the column labeled DT. If the data type of the field is variable, the notation "varies" will be displayed.

There are a number of data types defined by HL7. See section 2.15, "Data types"

2.5.3.4 Optionality

Definition: Whether the field is required, optional, or conditional in a segment.

In the segment attribute tables this information is provided in the column labeled OPT.

The designations for optionality are:

R

-

required

O

-

optional

C

-

conditional on the trigger event or on some other field(s). The field definitions following the segment attribute table should specify the algorithm that defines the conditionality for this field.

X

-

not used with this trigger event

B

-

left in for backward compatibility with previous versions of HL7. The field definitions following the segment attribute table should denote the optionality of the field for prior versions.

W

-

withdrawn

Note: For Versions 2.3 and higher: the optionality of fields should be explicitly documented in the segment field definitions that follow each segment definition table; if the optionality of fields within a segment changes depending on the trigger event, that optionality should also be explicitly documented.

For version 2.5 and higher, the optionality, table references, and lengths of data type components are supplied in component tables of the data type definition. The component definitions that follow the component table will elaborate on the optionality and table references. Where needed, additional detailed field definitions will follow the formal segment attribute tables. (See also Sections 2.5.4, "Message delimiters," 2.6, "Message construction rules," 2.15, "Data types").

2.5.3.5 Repetition

Definition: Whether the field may repeat. The value that appears in the repetitions column is the maximum number of allowed occurrences, e.g., a value of '3' would mean that the field can have '3 occurrences'; if unspecified, there is only one occurrence, i.e., cannot repeat.

In the segment attribute tables this information is provided in the column labeled RP/#.

The designations for Repetition are:

N or blank

-

no repetition

Y

-

the field may repeat an indefinite or site-determined number of times

(integer)

-

the field may repeat up to the number of times specified by the integer

Each occurrence may contain the number of characters specified by the field's maximum length. See Section 2.5.3.2, "Maximum length".

Usage Note: For improved readability some technical committees opt to leave the Repetition fields blank to indicate that the field may NOT repeat. A blank may NOT be construed to mean that the field may optionally repeat.

As of v2.5 the Repetition column is to be left blank if the field may NOT repeat.

2.5.3.6 Table

Note: The Vocabulary T.C. is the steward of this section.

Definition: The table attribute of the data field definition specifies the HL7 identifier for a set of coded values.

In the segment attribute tables, the table identifier is provided in the column labeled TBL#. If this attribute is not valued or blank, there is not a table of values defined for the field.

A number of conventions have been applied to this attribute of the data field definition.

1. If more than one table is applicable, the format xxxx/yyyy will be used to so designate multiple tables. Details on multiple tables will be specified in field or data type notes.

2. If the field is of data type ID or IS a table number will be allocated even if, in the case of IS, there may be a notation "No Suggested values".

3. If the field is of data type CF, CNE or CWE and one or more externally or locally defined tables may be used, the symbolic number 9999 will appear in the column. This is to indicate that table values are used, but no HL7/User-defined table can be allocated. The narrative may constrain which external tables can be used.

4. Tables embedded in field components or subcomponents will not be cited in the attribute column.

1. Tables embedded in data types are defined, save for the exceptions in point 2 below, in the data type's component table in section 2.A Data Types. They may, however, be constrained in the field note section. The field note definition supersedes the definition in the data type section.

2. Tables embedded in fields with a data type of CF, CNE, or CWE are only defined in the field notes section.

1. Values for HL7 tables shall not contain the embedded "suggested delimiters" delineated in section, 2.5.4, "Message delimiters".

HL7 defines four table types, reflecting content ownership: HL7 defined, User-defined, Imported and Externally defined. Local implementation may further constrain the table.

User-defined Tables: A user-defined table is a set of values that are locally or site defined. This accommodates certain fields, like PV1-3 - Assigned patient location, that will have values that vary from institution to institution. Even though these tables are not defined in the Standard, they are given a user-defined table number to facilitate implementations. HL7 sometimes publishes suggested values that a site may use as a starter set (e.g., Table 0001- Sex). The IS data type is often used to encode values for these tables. Note that some of these tables (e.g., Table 0302 - Point of care) may reference common master files.

There are some user-defined tables that contain values that might be standardized across institutions but for which no applicable official standard exists. For these a set of suggested values may be listed in Appendix A. These suggested values appear in the text in a standard box format (e.g., HL7 Table 0062 - Event Reason in Section 3.4.1.4, "Event reason code"). It is recommended that these values be used where applicable within an institution and serve as a basis for extensions as required. These values may, however, be redefined locally. The appropriate functional committee within HL7 solicits suggestions for additional values from institutions that are applying the Standard.

HL7 Tables: An HL7 table is a set of values defined and published by HL7. They are a part of the HL7 Standard because they affect the interpretation of the messages that contain them. These values may not be redefined locally; however, the table itself may be extended to accommodate locally defined values. This is particularly applicable in the case of HL7 table 0003 - Event Type. The ID data type is most often used to encode values for HL7 tables. The values are listed in Appendix A. These HL7 tables also appear in the text in a standard box format (e.g., HL7 table 0003 Event Type).

External Tables: An External table is a set of coded values defined and published by another standards organization. External tables are used to populate fields like FT1-19-Diagnosis Code - FT1. Another example is the encoding of clinical observations using LOINC codes. The CF, CNE and CWE data type are used to represent values for these fields.

External tables arise from applications where the concepts and possibly the codes are established by external agencies due to regulatory requirements or agreements between HL7 and other Standards Developing Organizations..

For user convenience, an External table will be assigned a number by HL7. Although the set of allowable codes (the code sets) are supplied by an external organization, the coding system code is assigned by HL7 in HL7 Table 0396 - Coding System. When the value is an HL7 extension to the table, the HL7 number should be used as the coding scheme.

The data type for the field will be CWE if 1) other tables are allowed in the field or 2) the external table may be locally extended or 3) when the code may be replaced by local text.

The data type for the field will be CNE if 1) no other table is allowed in the field and 2) the external table may not be locally extended and 3) text may not replace the code. A CNE field must have an HL7 defined or external table associated with it. A CNE field may be context sensitive such that a choice of explicit coding systems or value sets might be designated.

Imported Tables: An Imported table is a set of coded values defined and published by another standards organization. Imported tables are published by HL7 on behalf of other organizations. Their contents are not subject to approval by HL7 ballot. Such tables will be published with HL7 Standards. However, they may be updated more frequently than HL7 Standards.

Imported tables differ from External tables in that they have been imported into the HL7 standard subject to specific license or copyright requirements of the supplier/author. HL7 users will need to abide by the licensing and copyright requirements of the source when applicable.

For user convenience, an Imported table will be assigned a number by HL7. However, the coding system will be designated as that assigned by the supplier, and will be a member of HL7 Table 0396 - Coding System. An exception to this convention occurs when the value is an HL7 extension to the table; in that case the HL7 number should be used as the coding scheme.

Table 0292 - Vaccines administered is an example of an Imported table.

The data type rules apply as defined for the External table.

Local Tables: A local table is a table with a non-HL7 assigned table identifier and which contains a set of locally or site defined values. It may be locally assigned to local fields in Z segments or to HL7 fields having a CWE data type.

2.5.3.7 ID number

Definition: a small integer that uniquely identifies the data item throughout the Standard. In the segment definition this information is provided in the column labeled ITEM #.

2.5.3.8 Name

Definition: Descriptive name for the data item. In the segment attribute tables this information is provided in the column labeled ELEMENT NAME.

When the same name is used in more than one segment, it must have the same data type and semantic meaning in each segment as well as the same ID number. To deal with any ambiguities arising from this convention, whenever a field is referenced herein, the segment name and position must always be included.

2.5.4 Message delimiters

In constructing a message, certain special characters are used. They are the segment terminator, the field separator, the component separator, subcomponent separator, repetition separator, and escape character. The segment terminator is always a carriage return (in ASCII, a hex 0D). The other delimiters are defined in the MSH segment, with the field delimiter in the 4th character position, and the other delimiters occurring as in the field called Encoding Characters, which is the first field after the segment ID. The delimiter values used in the MSH segment are the delimiter values used throughout the entire message. In the absence of other considerations, HL7 recommends the suggested values found in Figure 2-1 delimiter values.

At any given site, the subset of the possible delimiters may be limited by negotiations between applications. This implies that the receiving applications will use the agreed upon delimiters, as they appear in the Message Header segment (MSH), to parse the message.

Note: The binary representation of the delimiter characters will vary with the character set used in the message.

Warning: Receivers should be made aware that while it is not best practice, the sender may omit the escape and subcomponent delimiters.

Figure 2-1. Delimiter values

This section addresses HL7 general rules for composing messages. Both the sender and receiver of the data must have predictable rules for how they will process the data. The reader is also referred to Section 2.B, "Conformance Using Message Profiles", where procedures for ensuring messaging integrity are discussed in detail.

Note: These message construction rules define the standard HL7 encoding rules, creating variable length delimited messages. Although only one set of encoding rules has been defined as a standard since HL7 Version 2.3, other encoding rules are possible (but since they are non-standard, they may only be used by a site-specific agreement).

2.6.1 Rules for the sender

2.6.1.1 Message Construction Pseudocode

procedure construct_message ( data ) {

identify_message_needed;

validate( data );

order_segments( data, segment_list );

foreach segment in ( segment_list ) {

insert segment.name; /* e.g., MSH */

/* gather all data for fields */

foreach field in ( fields_of( segment ) ) {

insert field separator; /* e.g., | */

/* gather occurrences (may be multiple only for fields that are allowed to repeat */

foreach occurrence in ( occurrences_of( field ) ) {

construct_occurrence( occurrence );

if not last ( populated occurrence ) insert repetition_separator; /* e.g., ~ */

}

break if last ( populated field );

}

insert segment_terminator; /* always<cr>! */

}

return;

}

procedure construct_occurrence ( occurrence ) {

/* gather populated components */

foreach component in ( components_of( occurrence ) ) {

get_subcomponent_data( component );

/* gather all data for subcomponents */

foreach subcomponent in ( subcomponents_of( component ) ) {

/* escape the field separator */

substitute( field_separator, \F\ );

/* escape the encoding characters */

substitute( component_separator, \S\ );

substitute( repetition_separator, \R\ );

substitute( escape_character, \E\ );

substitute( subcomponent_separator, \T\ );

insert subcomponent;

if not last ( populated subcomponent ) insert subcomponent_separator; /* e.g., & */

}

if not last ( populated component ) insert component_separator; /* e.g., ^ */

}

return;

}

2.6.1.2 Message Construction Flow Chart

The flow charts on the following pages represent another view of the message construction rules. The first shows the rules for transmitting a message; the second shows transmitting field occurrences.

2.6.2 Rules for the recipient

The following rules apply to receiving HL7 messages and converting their contents to data values:

1. ignore segments, fields, components, subcomponents, and extra repetitions of a field that are present but were not expected.

2. treat optional segments that were expected but are not present as consisting entirely of fields that are not present.

3. treat fields and components that are expected but were not included in a segment as not present.

2.6.3 Encoding rules notes

If a segment is to be continued across messages, use the extended encoding rules. These rules are defined in terms of the more general message continuation protocol (see Section 2.10.2, "Continuation messages and segments").

2.7 USE OF ESCAPE SEQUENCES IN TEXT FIELDS

2.7.1 Formatting codes

When a field of type TX, FT, or CF is being encoded, the escape character may be used to signal certain special characteristics of portions of the text field. The escape character is whatever display ASCII character is specified in the <escape character> component of MSH-2-encoding characters. For purposes of this section, the character \ will be used to represent the character so designated in a message. An escape sequence consists of the escape character followed by an escape code ID of one character, zero (0) or more data characters, and another occurrence of the escape character.

The escape sequences for field separator, component separator, subcomponent separator, repetition separator, and escape character are also valid within an ST data field.

The following escape sequences are defined:

No escape sequence may contain a nested escape sequence.

2.7.2 Escape sequences supporting multiple character sets

The following HL7 escape sequences are defined to support multiple character sets for fields, components and sub-components that are defined as data types FT, ST, and TX. They allow HL7 parsers to use escape codes (defined in the standards used below), without breaking, and without being non-conformant to the HL7 escape paradigm defined in this section.

\Cxxyy\ single-byte character set escape sequence with two hexadecimal values, xx and yy, that indicate the escape sequence defined for one of the character repertoires supported for the current message (i.e., ISO-IR xxx).

\Mxxyyzz\ multi-byte character set escape sequence with three hexadecimal values, xx, yy and zz. zz is optional.

Common character set escape sequences include the following which are defined in the standards mentioned:

Single-byte character sets:

Multi-byte codes:

In designating highlighting, the sending application is indicating that the characters that follow somehow should be made to stand out, but leaving the method of doing so to the receiving application. Depending on device characteristics and application style considerations, the receiving application may choose reverse video, boldface, underlining, blink, an alternate color or another means of highlighting the displayed data. For example the message fragment:

DSP| TOTAL CHOLESTEROL \H\240*\N\ [90 - 200]

might cause the following data to appear on a screen or report:

TOTAL CHOLESTEROL 240* [90 - 200]

whereas another system may choose to show the 240* in red.

2.7.4 Special character

The special character escape sequences (\F\, \S\, \R\, \T\, and \E\) allow the corresponding characters to be included in the data in a text field, though the actual characters are reserved. For example, the message fragment

DSP| TOTAL CHOLESTEROL 180 \F\90 - 200\F\

DSP| \S\----------------\S\

would cause the following information to be displayed, given suitable assignment of separators:

TOTAL CHOLESTEROL 180 |90 - 200|

^----------------^

2.7.5 Hexadecimal

When the hexadecimal escape sequence (\Xdddd...\) is used the X should be followed by 1 or more pairs of hexadecimal digits (0, 1, . . . , 9, A, . . . , F). Consecutive pairs of the hexadecimal digits represent 8-bit binary values. The interpretation of the data is entirely left to an agreement between the sending and receiving applications that is beyond the scope of this Standard.

2.7.6 Usage and Examples of Formatted Text

If the field is of the formatted text (FT) data type, formatting commands also may be surrounded by the escape character. Each command begins with the "." (period) character. The following formatting commands are available:

The component separator that marks each line defines the extent of the temporary indent command (.ti), and the beginning of each line in the no-wrap mode (.nf). Examples of formatting instructions that are NOT included in this data type include: width of display, position on page or screen, and type of output devices.

Figure 2-3 is an example of the FT data type from a radiology impression section of a radiology report:

Figure 2-3. Formatted text as transmitted

| \.in+4\\.ti-4\ 1. The cardiomediastinal silhouette is now within normal limits.\.br\\.ti-4\ 2. Lung fields show minimal ground glass appearance.\.br\\.ti-4\ 3. A loop of colon visible in the left upper quadrant is distinctly abnormal with the appearance of mucosal effacement suggesting colitis.\.in-4\|

Figure 2-4 shows one way of presenting the data in Figure 2-3. The receiving system can create many other interpretations by varying the right margin.

Figure 2-4. Formatted text in one possible presentation

The cardiomediastinal silhouette is now within normal limits.

Lung fields show minimal ground glass appearance.

A loop of colon visible in the left upper quadrant is distinctly abnormal with the appearance of mucosal effacement suggesting colitis.

2.7.7 Local

When the local escape sequence (\Zdddd...\) is used the Z should be followed by characters that are valid in a TX field. The interpretation of the data is entirely left to an agreement between the sending and receiving applications that is beyond the scope of this Standard.

2.8 VERSION COMPATIBILITY DEFINITION

The rules, described in section 2.6, "Message construction rules", for receiving HL7 messages and converting their contents to data values allow the following definition of a backward compatibility requirement between the 2.x versions of HL7:

Note: If an issue is not covered explicitly under these rules, no assumption should be made that the change is allowed.

The keys to understanding version compatibility are the following 2 axioms, plus the processing rules which state that unexpected information should be discarded.

• Old receivers receiving new messages should be able to continue receiving messages without error.

• New receivers should be able to understand old messages.

This section elaborates on what the kinds of changes can be done that satisfies these axioms. Only HL7 changes introduced in new versions are included. Local extensions are discussed in section 2.11, "Local Extension".

2.8.1 Adding messages or message constituents

A new message or a new constituent of an HL7 message may be introduced as described below. A sending system should be able to send a new message or new constituent; the receiver, regardless of its version level, must ignore any message or message constituent it is not expecting without generating an application failure. This does not preclude a receiver notifying the sender that additional element was ignored, but the receiving application should not fail just from the existence of additional element.

1. New messages may be introduced.

2. A new segment group may be defined.

3. The first segment in a newly-defined segment group, as of v 2.5, must be marked as required.

4. New segments may be introduced to an existing message. In general these will be introduced at the end of a message or a segment group, but they may be introduced elsewhere within the message if the segment hierarchy makes this necessary. Unless needed as a technical correction or for regulatory reporting purposes, a new segment may not be added to a deprecated message. As of v2.6 all new segments, except for those pertaining only to message transmission or control, must include an Action Code field as the first or second field as appropriate.

5. Care must be taken when introducing a new segment if this results in a situation in which a named segment X appears in two individual or group locations. See section 2.6, "Message Construction Rules".

6. New fields may be added at the end of a segment. A field that changes the semantic meaning of a segment (e.g., an Acton Code, or Mood code) may only be introduced in a pre-existing segment if the usage of the field is conditional on it not being used in messages with pre-existing trigger events. This is to avoid the risk of reversing the intent of the segment as it is known to the recipient of an earlier version. For example, if the Sender were to send the segment with a delete action code, the recipient would not understand that the information should be deleted.

7. A new data type may be introduced.

8. New components may be added at the end of a data type.

9. A new table may be introduced.

2.8.2 Changing messages or message constituents

Allowable changes to messages or message constituents can be categorized as name, data type, optionality, repeatability, length or definition changes.

1. The descriptive text name of a message or message constituent (except for segment group name) may be changed. This should have no impact on either the sender's ability to transmit a message or the receiver's ability to receive and understand the message. Reasons for changing the descriptive text name include: 1) clarify a misleading name, and 2) encompassing a broader use without jeopardizing current use.

2. The data type of a field or data type component may be changed. A sending system should be able to send the modified field or data type; the receiver, regardless of its version level, should be able to understand the message and to ignore any message constituent it is not expecting.

1. The data type of the field may be changed provided that the components of the new data type have the same structure and interpretation as the old data type. For example, an IS data type may be changed to a CE, but a PPN data type cannot be changed to a PN. An NM data type cannot be changed to an ST data type.

2. For existing fields in existing segments, data types may be changed if the leftmost (prior version) part of the field has the same meaning as it had in the prior version of HL7. This is in accordance with the rules governing the addition of new components and subcomponents described in the section above. In other words, if the new parts of the field (those that are part of the new data type) are ignored, what remains is the old field (defined by the old data type), which has the same meaning as it had in the prior version of HL7.

3. If a data type component has its data type changed, the structure and interpretation must remain the same as the pre-existing component. Any new component is added at the end of the data type.

1. The optionality of a message constituent may be changed. A sending system should be able to send the modified field; the receiver, regardless of its version level, should be able to understand the message. This pertains as follows:

1. Existing optional segment groups may be made required.

2. Existing optional segments may be made conditional or required.

3. Existing optional fields may be made conditional or required.

4. Existing required fields may be made conditional if a new trigger event has been applied. The condition must be specified such that the field remains required for the pre-existing trigger events.

5. Existing optional components of a data type may be made conditional or required.

1. The repeatability of a message constituent may be changed. A sending system should be able to send the modified message constituent; the receiver, regardless of its version level, should be able to understand the message. Note that if a non-repeating message constituent is made repeating, information sent in the new repetitions may be lost to the recipient who is not expecting them.

If HL7 has given, or will give, semantic meaning to the first instance, to allow backward compatibility, the first instance of the repeating constituent shall have the same meaning as the non-repeating constituent had in the prior version of HL7. In this way, a receiving application that interprets the message based upon the prior standard would continue to find the same intent communicated in the message.

If HL7 has not given, and will/can not give, semantic meaning to the first instance, and one or more implementation-applied business rules exist to select one of several occurrences to populate a non-repeating constituent, those same rules should be applied when a newer version of the standard allows for repetition of the constituent. By applying the prior business rules to determine the first occurrence of a repeating constituent, a receiving application that interprets the message based upon the prior standard would continue to find the same intent communicated in the message.

If, in the judgment of the owner/author of the standard section in question, changing a message constituent from non-repeating to repeating poses logical, parsing, business, or other compatibility issues, the owner/author may elect to create a new structure to eliminate the compatibility concern.

For example, if allowing a segment to repeat implies a change to the business intent of the message, the technical committee responsible can elect to define a new message structure (as a new message/trigger) and retain the old structure for backward compatibility.

This pertains as follows:

1. A segment group may change from non-repeating to repeating, subject to the backward compatibility concerns expressed above.

2. A segment group may NOT be changed from repeating to non-repeating.

3. A segment may be changed from non-repeating to repeating, subject to the backward compatibility concerns expressed above.

4. A segment may NOT be changed from repeating to non-repeating.

5. A field may be changed from non-repeating to repeating, subject to the backward compatibility concerns expressed above. A field may NOT be changed from repeating to non-repeating.

1. The length of a field, data type or data type component may be increased.

2. Table definition may change.

1. A table may be changed from user-defined to HL7 defined or externally defined.

2. A table may be changed from HL7 defined to an externally defined table. When this occurs, the data type of the field should be changed to a CNE or CWE.

2.8.3 Deprecating messages or message constituents

Any required, optional or conditional constituent of an HL7 message, including the message itself, may be deprecated. This means that one of the following situations has occurred:

• The message or message constituent no longer has a meaningful purpose

• The message or message constituent has been replaced by a better method

Language will be inserted stating the fact of deprecation, the version in which the deprecation occurred, and what message or message constituent, if any, replaces it. The phrase "Retained for backward compatibility only in version 2.x; refer to section n.m instead" will be the standard language for such an occurrence.

The fact of deprecation should not affect either the sender or the receiver because the message or message constituent is retained for backward compatibility. Implementers, by site agreement, may agree to not support deprecated message constituents.

The following are allowed:

1. A message may be deprecated.

2. A trigger event may be deprecated.

3. A message structure may be deprecated.

4. A segment in an existing message may be deprecated. Implementers, by site agreement, may agree to not support deprecated segments. If the segment that is to be deprecated has dependents the entire segment group must be deprecated. For example, in a group [{ABC[DEF][{GHI}]], DEF and/or GHI may be deprecated, but ABC cannot be deprecated without deprecating the whole.

5. A field may be deprecated by HL7. Implementers, by site agreement, may agree to not use deprecated fields.

6. A data type may be deprecated provided all fields referencing it have been deprecated or there is an explicit statement that the data type is not to be used in any field defined in the future.

7. A data type component may be deprecated.

8. A table may be deprecated. This includes HL7 tables, user-defined tables, imported external tables and reference to external tables.

9. An entry in an HL7-defined table may be deprecated. The table itself should be reviewed if it contains a substantial number of deprecated members.

10. An entry in an imported external table may not be deprecated.

2.8.4 Removing messages or message constituents

A message or message constituent may be removed from the standard when criteria described in this section are met. HL7 will track old names so they are not re-used.

Note: To refer to the detail of a withdrawn message constituent, the reader will need to review the appropriate earlier version of the standard. By site agreement senders and receivers may agree to continue to use messages and/or message constituents that have been removed.

1. A message constituent may be immediately removed from the standard based on the following criteria (immediately means in the same version in which the criteria are met.).

1. A message structure may be removed immediately provided no message references it in the standard. Care must be taken lest a message structure is prematurely removed if the associated trigger event that contributed to its name is removed. For example, if a message structure ABC_D01 is associated with trigger events D01, D02 and D03 and D01 is changed and becomes associated with another existing message structure DEF_E01, the message structure ABC_D01 is still active and valid for trigger events D02 and D03.

2. A segment may be removed immediately provided no message references it in the standard.

3. A data type may be removed immediately provided no fields reference it. This occurs when the data type for a field is changed to a new data type that incorporates the components of the old one.

4. A table may be removed provided all fields and components, where the table has been used have been removed. This applies to HL7, user-defined and external tables. It is recognized that this might have a ripple effect.

1. A message constituent, except as noted in points c, d and e below, will be withdrawn and removed, no sooner than, after 2 versions in a deprecated state. For example, if a message was originally deprecated in v 2.3.1, its definition can be removed when v 2.6 is published.

1. A message type and its definition may be removed.

2. A trigger event and its definition may be removed.

3. A segment group in an existing message may be removed.

4. A segment in an existing message may be removed.

1. A deprecated field in an existing segment may NOT be removed from the standard. However, no sooner than, after 2 versions in a deprecated state, the field will be marked as withdrawn and all explanatory narrative will be removed

2. A deprecated component in an existing data type may NOT be removed from the standard. However, no sooner than, after 2 versions in a deprecated state, the component will be marked as withdrawn and all explanatory narrative will be removed.

3. A deprecated member of an existing HL7 table may NOT be removed from the standard. However, no sooner than, after 2 versions in a deprecated state, the table member will be marked as withdrawn and all explanatory narrative will be removed from the description and comment column.

2.8.5 Early adoption of HL7 changes

Early adoption of HL7 changes that have been approved by the technical committee for the next membership ballot is a common practice and is not prohibited, but carries risk. Such changes may be rejected or modified in the balloting process. One example is that the change may pass but may be positioned differently in the segment or data type.

2.8.6 Technical correction rules

Technical corrections may be applied between versions on a case-by-case basis. These corrections will be published on the HL7 website. The following meet criteria for technical correction:

1. Spelling correction

2. Incorrect section reference

3. Transcription error in an imported external table

4. Correction of an inconsistency between a segment attribute table and the field narrative

5. Erroneous examples

6. Erroneous/misleading descriptions

2.9 MESSAGE PROCESSING RULES

The processing rules described here apply to all exchanges of messages, whether or not the HL7 encoding rules or Lower Layer Protocols are used. They represent the primary message processing mode. The user may use either the original processing rules, described in section 2.9.2 or the enhanced processing rules, described in section 2.9.3.

Note: The MCF - Delayed Acknowledgement message has been removed from the standard. It was deprecated in v 2.2. Accordingly, the narrative notes regarding deferred processing have been removed from this section.

Certain variants exist and are documented elsewhere:

1. an optional sequence number protocol. Refer to section 2.10.1.

2. an optional protocol for continuing a very long message. Refer to section 2.10.2.

Because the protocol describes an exchange of messages, it is described in terms of two entities, the initiating and responding systems. Each is both a sender and receiver of messages. The initiating system sends first and then receives, while the responding system receives and then sends.

In overview this exchange proceeds as follows:

Message Exchange

The initiating application creates a message with data values as defined in the appropriate chapter of this Standard. The fields shown below should be valued in the MSH segment (as defined under the MSH segment definition of this chapter). The message is encoded according to the applicable rules and sent to the lower level protocols, which will attempt to deliver it to the responding application. (For definitions of the MSH fields see Section 2.14.9, "MSH - message header segment")

Certain other fields in the MSH segment are required for the operation of the HL7 encoding rules; they will not be relevant if other encoding rules are employed.

The event code in the second component of MSH-9-message type is redundantly shown elsewhere in some messages. For example, the same information is in the EVN segment of the ADT message. This is for compatibility with prior versions of the HL7 protocol. Newly defined messages should only show the event code in MSH-9-message type.

2.9.2 Message response using the original processing rules

2.9.2.1 Accept and validate the message in responding system

Upon receipt of the message, when the Original Acknowledgement rules are used, the protocol software in the responding system validates it against at least the following criteria:

Note: Both MSH-15 - accept acknowledgment type and MSH-16 - application acknowledgment type are null or not present.

1. the value in MSH-9-message type is one that is acceptable to the receiver.

2. the value in MSH-12-version ID is acceptable to the receiver.

3. the value in MSH-11-processing ID is appropriate for the application process handling the message.

If any of these edits fail, the protocol software rejects the message. That is, it creates an ACK message with AR in MSA-1-acknowledgment code.

If successful, the process moves to the next step.

2.9.2.2 Accept and validate/process the message in the receiving application

Upon successful validation by the responding system, the message is passed to the receiving application, which performs one of these functions:

1. process the message successfully, generating the functional response message with a value of AA in MSA-1-acknowledgment code.

2. send an error response, providing error information in functional segments to be included in the response message with a value of AE in MSA-1-acknowledgment code.

3. fail to process (reject) the message for reasons unrelated to its content or format (system down, internal error, etc.). For most such problems it is likely that the responding system will be able to accept the same message at a later time. The implementers must decide on an application-specific basis whether the message should be automatically sent again. The response message contains a value of AR in MSA-1-acknowledgment code.

The MSH segment in the response is constructed anew following the rules used to create the initial message described above. In particular, MSH-7-date/time of message and MSH-10-message control ID refer to the response message; they are not echoes of the fields in the initial message. MSH-5-receiving application, MSH-6-receiving facility, and MSH-11-processing ID contain codes that are copied from MSH-3-sending application, MSH-4-sending facility and MSH-11-processing ID in the initiating message.

In all the responses described above, the following values are put in the MSA segment. Note that the field definitions for the MSA segment fields are in Section 2.14.8.

The receiving application then passes the response message back to the responding system for the next step in the process.

2.9.2.3 Transmit the response message

Upon receiving the response message from the receiving application, the responding system transmits it to the initiating system.

The initiator processes the response message.

2.9.3 Response using enhanced acknowledgement

1. the responding system receives the message and commits it to safe storage. This means that the responding system accepts the responsibility for the message in a manner that releases the sending system from any obligation to resend the message. The responding system now checks the message header record to determine whether or not the initiating system requires an accept acknowledgment message indicating successful receipt and secure storage of the message. If it does, the accept acknowledgment message is constructed and returned to the initiator.

2. at this point, the requirements of the applications involved in the interface determine whether or not more information needs to be exchanged. This exchange is referred to as an application acknowledgment and includes information ranging from simple validation to a complex application-dependent response. If the receiving system is expected to return application-dependent information, it initiates another exchange when this information is available. This time, the roles of initiator and responder are reversed.

2.9.3.1 Accept and validate the message in responding system

Upon receipt of the message, when the Enhanced Acknowledgement rules are used, the protocol software in the responding system makes an initial determination as to whether or not the message can be accepted, based on factors such as:

Note: At least one of MSH-15-accept acknowledgment type or MSH-16-application acknowledgment type is not null.

1. the status of the interface

2. the availability of safe storage onto which the message can be saved

3. the syntactical correctness of the message, if the design of the receiving system includes this type of validation at this phase

4. the values of MSH-9-message type, MSH-12-version ID, and MSH-11-processing ID, if the design of the receiving system includes this type of validation at this phase

It then examines the Message Header segment (MSH) to determine whether or not the initiating system requires an accept acknowledgment.

2.9.3.2 Transmit general acknowledgement message

A general acknowledgement message is not always required by the initiating system, but if it is the responding system sends one of the following:

1. a commit accept (CA) in MSA-1-acknowledgment code if the message can be accepted for processing

2. a commit reject (CR) in MSA-1-acknowledgment code if the one of the values of MSH-9-message type, MSH-12-version ID or MSH-11-processing ID is not acceptable to the receiving application

3. a commit error (CE) in MSA-1-acknowledgment code if the message cannot be accepted for any other reason (e.g., sequence number error)

The MSH segment in the response is constructed anew following the rules used to create the initial message described above. In particular, MSH-7-date/time of message and MSH-10-message control ID refer to the response message; they are not echoes of the fields in the initial message. MSH-5-receiving application, MSH-6-receiving facility, and MSH-11-processing ID contain codes that are copied from MSH-3-sending application, MSH-4-sending facility and MSH-11-processing ID in the initiating message.

For this response, the following values are put in the MSA segment. Note that the field definitions for the MSA segment fields are in Section 2.14.8, 'MSA - message acknowledgment segment":

Note: MSH-15-accept acknowledgment type and MSH-16-application acknowledgment type are not valued (not present or null). At this point, the accept portion of this message exchange is considered complete.

2.9.3.3 Transmit application acknowledgement

If the message header segment indicates that the initiating system also requires an application acknowledgment, this will be returned as the initial message of a later exchange.

For this message, the receiving system acts as the initiator. Since the message it sends is application-specific, the layouts of these application-level response messages are defined in the relevant application-specific chapter. If needed, this application acknowledgment message can itself require (in MSH-15-accept acknowledgment type) an accept acknowledgment message (MSA). MSH-16-application acknowledgment type, however, is always null, since the protocol does not allow the application acknowledgment message to have an application acknowledgment.

For this response, the following values are put in the MSA segment. Note that the field definitions for the MSA segment fields are in Section 2.14.8, "MSA - message acknowledgment segment".

At this point, the application acknowledgment portion of this message exchange is considered complete.

If the processing on the receiving system goes through multiple stages, chapter-defined messages may be used to relay status or informational changes to other systems (including the original initiating system). Such messages are not part of the acknowledgment scheme for the original message, but are considered to be independent messages triggered by events on the (original) responding system.

Note: The original acknowledgment protocol is equivalent to the enhanced acknowledgment protocol with MSH-15-accept acknowledgment type = NE and MSH-16-application acknowledgment type = AL, and with the application acknowledgment message defined so that it never requires an accept acknowledgment (MSH-15-accept acknowledgment type = NE).

2.10 SPECIAL HL7 PROTOCOLS

This section contains several extensions to the basic HL7 message protocol. These extensions represent implementation choices, and are to be used on a site-specific and application-specific basis as needed.

2.10.1 Sequence number protocol

For certain types of data transactions between systems the issue of keeping databases synchronized is critical. An example is an ancillary system such as lab, which needs to know the locations of all inpatients to route stat results correctly. If the lab receives an ADT transaction out of sequence, the census/location information may be incorrect. Although it is true that a simple one-to-one acknowledgment scheme can prevent out-of-sequence transactions between any two systems, only the use of sequence numbers can prevent duplicate transactions.

Note: Although this sequence number protocol is limited to the use of sequence numbers on a single transaction stream between two applications, this sequencing protocol is sufficiently robust to allow the design of HL7-compatible store-and-forward applications.

1. initial conditions:

1. the system receiving the data stream is expected to store the sequence number of the most recently accepted transaction in a secure fashion before acknowledging that transaction. This stored sequence number allows comparison with the next transaction's sequence number, and the implementation of fault-tolerant restart capabilities.

2. the initiating system keeps a queue of outgoing transactions indexed by the sequence number. The length of this queue must be negotiated as part of the design process for a given link. The minimum length for this queue is one.

3. the sequence number is a positive (non-zero) integer; and it is incremented by one (by the initiating system) for each successive transaction.

1. starting the link:

1. the value of 0 (zero) for a sequence number is reserved: it is allowed only when the initiating system (re-)starts the link.

2. if the receiving system gets a transaction with a 0 (zero) in the sequence number field, it should respond with a general acknowledgment message whose MSA contains a sequence number one greater than the sequence number of the last transaction it accepted in the Expected Sequence Number field. If this value does not exist (as on the first startup of a given link), the MSA should contain a sequence number of -1, meaning that the receiving system will use the positive, non-zero sequence number of the first transaction it accepts as its initial sequence number (see re-synching the link, item e below).

3. the initiating system then sends the transaction indexed by the expected sequence number (if that expected transaction is still on its queue). Otherwise the link is frozen until an operator intervenes.

1. normal operation of the link:

As it accepts each transaction, the receiving system securely stores the sequence number (which agrees with its expected sequence number), and then acknowledges the message by echoing the sequence number in MSA-4-expected sequence number.

1. error conditions (from point of view of initiating system). These are generated by the receiving system, by its comparison of the sequence number sent out (with the MSH in MSH-13-sequence number) with the expected sequence number (MSA-4-expected sequence number received with the MSA).

1. expected sequence number is one greater than current value. The previous acknowledgment was lost. That transaction was sent again. Correct by sending next transaction.

2. expected sequence number less than current value. Initiating system can try starting again by issuing a transaction with a sequence number of zero; or freeze the link for operator intervention.

3. other errors: freeze the link for operator intervention

1. forcing resynchronization of sequence numbers across the link. The value of -1 for a sequence number is reserved: it is allowed only when the initiating system is re-synchronizing the link. Thus if the receiving system gets a value of -1 in the sequence number field, it should return a general acknowledgment message with a -1 in the expected sequence number field. The receiving system then resets its sequence number, using the non-zero positive sequence number of the next transaction it accepts.

2. notes

When the initiating system sends a message with a sequence number of 0 or -1 (see b or e above), the segments beyond the MSH need not be present in the message, or, if present, all fields can be null. In terms of the responding system, for these two cases, only a General acknowledgment message is needed.

2.10.2 Continuation messages and segments

Sometimes, implementation limitations require that large messages or segments be broken into manageable chunks. We use the term "fragmentation" to describe how a logical message is broken into one or more separate HL7 messages. HL7 consciously identifies two situations where this may happen.

• First, a single segment may be too large. HL7 uses the "ADD" segment to handle breaking a single segment into several smaller segments.

• Second, a single HL7 message may be too large. HL7 uses the DSC segment and the continuation protocol to handle message fragmentation.

Note: HL7 does not define what "too large" means. Acceptable values are subject to site negotiations.

See chapter 5 for a discussion of the continuation pointer segment and the continuation pointer field, and their use in the continuation of responses to queries and in the continuation of unsolicited update messages.

2.10.2.0 hiddentex

2.10.2.1 Segment fragmentation/continuation using the ADD segment

Beginning with version 2.4, the ADD segment can be used within a message to break a long segment into shorter segments within a single HL7 message.

Note: Unless some explicit agreement exists between systems, a receiving application should not infer semantic meaning from the placement of the ADD segment.

To break a large segment,

1. the segment being continued (call it ANY for this example) is ended at an arbitrary character position and terminated with the standard segment terminator (carriage return).

2. the following segment is the ADD segment. All characters after the ADD and field separator ("|") are logically part of the preceding segment. All succeeding consecutive ADD segments contribute characters to the ANY segment until a non ADD segment is found.

3. an ADD segment with no field separator takes on special meaning. See Section 2.10.2.3, "Segment fragmentation across messages".

For example, segment "C" can be fragmented within an HL7 message as follows:

A|1

B|2

C|34

ADD|5|678|

ADD|90

D|1

This is logically the same as

A|1

B|2

C|345|678|90

D|1

Note that the "|" at the end of the first ADD segment is part of the value, while the first "|" of each ADD is not.

2.10.2.2 Segment fragmentation/continuation using the DSC segment

When a message itself must be fragmented and sent as several HL7 messages, the DSC segment is used.

1. First, the logical message is broken after an arbitrary segment.

2. Next, a DSC segment is sent. The DSC-1-Continuation pointer field will contain a unique value that is used to match a subsequent message with this specific value.

3. The DSC terminates the first fragment of the logical message.

4. A subsequent message will contain in MSH-14-Continuation pointer, a value that matches the value from DSC-1. (The presence of a value in MSH-14 indicates that the message is a fragment of an earlier message.). Each subsequent message will have its own unique value for MSH-10-Message control ID. Coordination between DSC-1-Continuation pointer and the subsequent message's MSH-14-Continuation pointer is used to link the fragments in their proper order.

5. The logical message is the concatenation of the contents of the first message (which while having no value in MSH-14, did end with DSC, and hence was actually a message fragment), plus all subsequent fragments (as identified by values in MSH-14).

6. If enhanced mode acknowledgments are used to request an accept ACK, then the receiver will acknowledge each fragment with an ACK message. Since each fragment has its own Message Control ID, each fragment level accept ACK will use the Message Control ID from the fragment it is acknowledging.

7. If enhanced mode acknowledgments are used to request an application level ACK, then the receiver will send an acknowledgment after receiving the final fragment.

Note: The application level ACK should refer to the message by the Message Control ID of the first fragment.

Note: The receiver can tell that a given incoming message is a fragment by the presence of the trailing DSC. Subsequent HL7 messages are identified as fragments by the presence of an MSH-14 value. The presence of a DSC in a fragment indicates that more fragments are to follow.

It is a protocol error to end a message with DSC, and then never send a fragment.

For example, a single logical message may be fragmented into three HL7 messages:

---- Sender HL7 message (incomplete,fragment1)---

MSH|||||||||1001||2.4|123||..

A|...

B|...

DSC|W4xy

---- Sender HL7 message (fragment 2)---

MSH|||||||||2106||2.4|124|W4xy|

C|...

D|...

DSC|V292

----- another HL7 message(fragment 3, final)---

MSH|||||||||2401||2.4|125|V292

E|...

Such a sequence is logically the same as the single message:

MSH|...|2.4|123||..

A|...

B|...

C|...

D|...

E|...

See example in section 2.17.4 for a more elaborate example.

2.10.2.3 Segment fragmentation across messages

If the last segment of a fragment itself needs to be broken, then the following idiomatic use of ADD shall apply.

1. the segment being continued (call it ANY for this example) is ended at an arbitrary character position and terminated with the standard segment terminator (carriage return).

2. the following segment is the ADD segment. It will contain no characters other than "ADD". (The lack of characters signals the receiver that ANY will be continued.)

3. The second following segment will be the DSC, used as described above in Section 2.10.2.2, "Segment fragmentation/continuation using the DSC segment".

4. The first segment of the following fragment will be an ADD segment. The characters of this ADD segment are logically part of the ANY segment of the previous fragment.

For example

MSH|...|2.4|

ANY|12

ADD

DSC|JR97

--------- (fragment 2)

MSH|...|2.4|JR97

ADD|345

is logically the same as

MSH|...|2.4

ANY|12345

1. transaction flow for a continued unsolicited message with a continued segment.

First unsolicited message and acknowledgment:

Second unsolicited message and acknowledgment:

Note: This second message could itself be continued with a second DSC and (if needed) a second ADD segment prior to it.

There are instances when it is convenient to transfer a batch of HL7 messages. Common examples would be a batch of financial posting detail transactions (DFT's) sent from an ancillary to a financial system. Such a batch could be sent online using a common file transfer protocol, or offline via tape or diskette.

2.10.3.0

2.10.3.1 HL7 batch file structure

The structure of an HL7 batch file is given by the following (using the HL7 abstract message syntax)

Notes:

The sequence numbering protocol has a natural application in batch transfers. See the discussion of batch acknowledgments that follows.

Although a batch will usually consist of a single type of message, there is nothing in the definition that restricts a batch to only one message type.

The HL7 file and batch header and trailer segments are defined in exactly the same manner as the HL7 message segments. Hence the HL7 message construction rules of Section 2.6, "Message construction rules," can be used to encode and decode HL7 batch files.

There are only two cases in which an HL7 batch file may contain zero HL7 messages:

1. a batch containing zero HL7 messages may be sent to meet a requirement for periodic submission of batches when there are no messages to send.

2. a batch containing zero negative acknowledgment messages may be sent to indicate that all the HL7 messages contained in the batch being acknowledged are implicitly acknowledged. See Section 2.10.3.3, "Acknowledging batches."

2.10.3.2 Related segments and data usage

The following segments relate to the HL7 Batch Protocol:

• BHS Batch Header (See section 2.14.2)

• BTS Batch Trailer (See section 2.14.3)

• FHS File Header (See section 2.14.6)

• FTS File Trailer (See section 2.14.7)

The BTS segment contains a field, BTS-3-batch totals, which may have one or more totals drawn from fields within the individual messages. The method for computing such totals will be determined on a site or application basis unless explicitly stated in a functional chapter.

2.10.3.3 Acknowledging batches

In general, the utility of sending batches of data is that the data is accepted all at once, with errors processed on an exception basis. However, it is a permissible application of HL7 to acknowledge all messages. Several options for acknowledgment are given and will be chosen on an application basis. In these cases, the sequence numbering protocol can be useful to the applications processing the batch.

The options are:

1. all messages are acknowledged in the response batch.

2. the receiving system prints some form of batch control report, which is then dealt with manually by personnel at the sending system. No acknowledgments are performed by the protocol software.

3. an automated acknowledgment batch is created containing acknowledgment messages only for those messages containing errors. In this mode an empty acknowledgment batch may be created (i.e., an HL7 batch file without any HL7 acknowledgment messages).

In each case where there is a response batch, its format is a batch of individual messages. Each individual message is in the format defined for an online response in the chapters. Consider, for example, a batch that might be constructed to respond to a batch of Detailed Financial Transactions (Chapter 6). The messages in the response batch would consist entirely of ACK messages, since ACK is the response shown in Chapter 6.

When batches are retransmitted after the correction of errors, BHS-12-reference batch control ID should contain the batch control ID of the original batch.

2.10.3.4 Batch message as a query response

NOTE: The QRD and QRF segments were retained for backward compatibility only as of v 2.4. The reader is referred to chapter 5, section 5.4, for the current query/response message structure.

The HL7 query also can be used to query for a batch in the following manner:

1. use the value BB or BL of QRD-5-deferred response type to specify a batch response. The query will be acknowledged with a general acknowledgment as in the Deferred Access example above (see chapter 5)

2. in addition, insert into the batch file the QRD and QRF segments as follows:

1. the acknowledgment of a batch is described in this chapter (see Section 2.10.3.3, "Acknowledging batches").

2.10.4 Protocol for interpreting repeating segments or segment groups in an update Message

This section describes the protocol for interpreting repeating segments or segment groups in an update message. Common examples of repeating segments are NK1 and OBX shown as [{NK1}] and [{OBX}] in the abstract message syntax. Common examples of segment groups are displayed as {ORC RXO [{RXC}]} or [{IN1 [IN2] [{IN3}]}] in the abstract message syntax

There are 2 methods of update: the "snapshot" and the "action code/unique identifier" modes. These are defined in sections 2.10.4.1 and 2.10.4.2 below.

If a particular repeating segment can be updated by either of these two modes, the parties concerned will determine by agreement among messaging partners whether an interface will use the "snapshot" mode or the "action code/unique identifier" mode.

Both the sender and receiver of the data must have predictable rules for how they will process the data in repeating segments or segment groups regardless of which mode is used. This should be documented in the Conformance Profile. It is critical to know, for instance, if the Sender is the System of Record.

2.10.4.0

2.10.4.1 Snapshot mode update definition

For segments that do not contain unique identifiers and action codes (mainly NTE and patient administration segments), the only option is to treat the information in the repeating segments and segment groups as a whole.

When an HL7 abstract message syntax includes these repeating units or sets, there is no implicit indication of how they interact with a similar set in a prior or subsequent message. Interpretation is not obvious from the message syntax particularly if the requirement is to update only part of the information previously sent.

The existence of a segment, and possibly the lack of existence of a segment, may serve to add, update, replace, or delete information passed in similar segments in prior messages. Special consideration is warranted in the case where multiple instances of a segment exist in a message.

In the "snapshot" mode, the information contained in the set of repeating segments or segment groups from the incoming message replaces the corresponding information in the receiving application. This is equivalent to a deletion of the prior information followed by the addition of the newly supplied information. In this mode, everything (all repeating segments and segment groups) must be sent with every subsequent message in the series of messages. There is no other way to indicate which ones changed and which ones did not.

To specify "delete all of the segments in this repeating group" in the snapshot mode, send a single segment with "delete data" (indicated by a value of "") in all fields. This actively signals the receiver that there is information that needs to be deleted. If no segment were sent, this would equate to "no information." No information should not signal the receiver to take an action. There would be risk that the receiver might misinterpret the sender's intent.

To support assertions made in some chapters, e.g., chapter 6, and common practice at implementation sites, as of v2.6, the signal methods have been extended. By agreement among messaging partners or Conformance Profile, a sender may opt to signal deletion of data in the following manner:

Transmit the null value only:

• in the key identifier field if the segment has an explicit one - all other fields have no data

• in the first field of the segment to indicate that all are to be deleted

• in any combination of fields that the Sender customarily sends to the recipient - all other fields have no data

• in all required fields all - other fields have no data

This obviates the need for the Sender to populate fields ordinarily not sent and not expected by the receiver.

2.10.4.1.1 Snapshot Mode and Repeating Segments - Example

Example A: if a patient record indicated a 2 sisters and a brother as next of kin, this would be represented as follows in the add person/patient information message:

MSH||||||||ADT^A28^ADT_A05|...<cr>

EVN|...<cr>

PID|...<cr>

NK1|1|Nuclear^Nancy^D|SIS^Sister^HL70063|...<cr>

NK1|2|Nuclear^Nelda^W|SIS^Sister^HL70063|...<cr>

NK1|3|Nuclear^Neville^S|BRO^Brother^HL70063|...<cr>

PV1|...<cr>

If, subsequently, the one of the sisters was delisted as next of kin, it would be necessary to send both the remaining "brother" and "sister" records in order to form a complete replacement set in an update person information message:

MSH|||||||||ADT^A31^ADT_A05|...<cr>

EVN|...<cr>

PID|...<cr>

NK1|1|Nuclear^Nancy^D|SIS^Sister^HL70063|...<cr>

NK1|2|Nuclear^Neville^S|BRO^Brother^HL70063|...<cr>

PV1|...<cr>

If all next of kin were to be subsequently delisted, an update message with a single null populated segment would instruct the receiving system to delete information represented by any prior set:

MSH||||||||ADT^A31^ADT_A05|...<cr>

EVN|...

PID|...

NK1|""|""|""|""|<cr>

PV1|...<cr>

Alternatively, as of v2.6, the deletion could be signaled by sending a Null in the first field of the NK1 segment. This is its only required field.

MSH||||||||ADT^A31^ADT_A05|...<cr>

EVN|...

PID|...

NK1|""|<cr>

PV1|...<cr>

2.10.4.1.2 Snapshot Mode and Repeating Segment Groups

Treatment of the repeating segment group is analogous to the handling of the repeating segment described above. To indicate deletion of all of the information in a repeating segment group, it is only necessary to delete the anchoring segment of the segment group. This is accomplished just as described above for deleting a repeating segment. This pertains to segments governed by snapshot mode, not action code.

Example: An account is created for Adam Everyman. He is insured under plan ID A357 with an insurance company known to both systems as BCMD, with a company ID of 1234. He is also covered by his wife's insurance under plan ID A789 with an insurance company known to both systems as VGMC, with a company ID of 6789.

MSH||||||||BAR^P01^BAR_P01|...<cr>

EVN|

PID|

IN1|1|A357|1234|BCMD

IN2|

IN3|

IN1|2|A789|6789|VGMC

IN2|

IN3|

Subsequently it is learned that his wife has changed insurance plans. Her new plan is now C45. The insurance company and company ID have remained the same. A BAR^P05 might be sent.

MSH||||||||BAR^P05^BAR_P05|...<cr>

EVN|

PID|

IN1|1|A357|1234|BCMD

IN2|

IN3|

IN1|2|C45|6789|VGMC

IN2|

IN3|

It is later discovered that the patient is not covered by either plan and now has no insurance at all. A BAR^P05 is again sent. In accordance with chapter 6, this can be signaled by showing Null in the plan field.

MSH||||||||BAR^P05^BAR_P05|...<cr>

EVN|

PID|

IN1|""|""

If, on the other hand, the patient still had his coverage, and only the wife's insurance had been dropped, a fully populated IN1 segment group would be transmitted. The presence of only one IN1 in a subsequent message conveys the "full group replacement" notion. The BAR^P05 would be transmitted and would be interpreted to mean "retain plan A357; delete and other plans":

MSH||||||||BAR^P05^BAR_P05|...<cr>

EVN|

PID|

IN1|1|A357|1234|BCMD

IN2|

IN3|

2.10.4.2 Action code/unique identifier mode update definition

In the "action code/unique identifier" mode (action code mode), each member of a segment or segment group can be acted upon independently of the other members. Thus, it is possible to delete or update a member of the set without including the other members of the set. The choice of delete/update/insert is determined by the action code (or an equivalent such as result status in an ORU Observation Report message). Refer to HL7 Table 0206 - Segment action code for valid values.

HL7 Table 0206 - Segment action code

The unique identifier unambiguously identifies one of multiple repetitions of the repeating segment or segment group in a way that does not change over time. It is not dependent on any particular message identifier level (MSH) fields; it functions across messages, not just within a message. For a single segment repetition, the unique identifier may be an explicit field (e.g., IAM-7 Allergy Unique Identifier) or a combination of fields (IAM suggests IAM-3 Allergen Identifier in the context of the particular patient). For a repeating segment group, an identifier in the anchoring segment would identify the repeating set. For MFN messages, MFI-1 Master File Identifier and MFE-4 Primary Key Value identify the particular table and record.

Example 1: If a patient is allergic to penicillin and shellfish, the following message would be sent showing an Action code of "A(dd) in IAM-6:

MSH|||||||||ADT^A60^ADT_A60|...<cr>

EVN|...<cr>

PID|...<cr>

IAM|1||peni|||A<cr>

IAM|2||shell||A<cr>

Subsequently, if it is learned that the patient is not allergic to shellfish, the following message would be sent showing an Action code of "D(elete) in IAM-6:

MSH|||||||||ADT^A60^ADT_A60|...<cr>

EVN|...<cr>

PID|...<cr>

IAM|1||shell||D<cr>

Some messages, Orders and Observations, in particular, do not use table 0206. Order control codes are used to unambiguously specify the action to be taken.

Example 2: if a set of orders had been sent as

MSH|||||||||OML^O21^OML_O21|...<cr>

PID|...

ORC|NW|987654^CIS|...<cr>

ORC|NW|876543^CIS|...<cr>

ORC|NW|765432^CIS|...<cr>

and subsequently order 876543 was cancelled, the following message would target that specific segment instance without affecting the other orders. ORC-1 contains order control code "CA" for cancel. ORC-2 identifies the specific order number.

MSH|||||||||OML^O21^ OML_O21|...<cr>

PID|

ORC|CA|876543^CIS|...<cr>

Example 3: Add staff person to Provider master:

MSH|^~\&|HL7REG|UH|HL7LAB|CH|200102280700||MFN^M02^MFN_M02|MSGID002|P|2.5|||AL|NE

MFI|PRA^Practitioner Master File^HL70175||UPD|||AL

MFE|MAD|U2246|200102280700|PMF98123789182^^PLW

STF|PMF98123789182^^PLW|U2246^^^PLW |SEVEN^HENRY^L^JR^DR^M.D.|P|M|19511004|A|^ICU|^MED|(555)555-1002X345CO~(955)555-1002CH(206)689-1345X789CB|1002 Healthcare Drive^SUITE 227^AnnArbor^MI^48104^H~1012 Healthcare Drive^^AnnArbor, MI^48104^O |19890125^GHH&Good Health Hospital&L01||PMF88123453334|74160.2326@COMPUSERV.COM|B

The birth date was discovered to be in error. An MFN^M02 message is sent with the MFE-1 having a value of MUP for Update Record for master File. The corrected birth date (19521004) appears in STF-6:

MSH|^~\&|HL7REG|UH|HL7LAB|CH|200102280700||MFN^M02^MFN_M02|MSGID002|P|2.5|||AL|NE

MFI|PRA^Practitioner Master File^HL70175||UPD|||AL

MFE|MUP|U2246|200102280700|PMF98123789182^^PLW

STF|PMF98123789182^^PLW|U2246^^^PLW |SEVEN^HENRY^L^JR^DR^M.D.|P|M|19521004|A|^ICU|^MED|(555)555-1002X345CO~(955)555-1002CH(206)689-1345X789CB|1002 Healthcare Drive^SUITE 227^AnnArbor^MI^48104^H~1012 Healthcare Drive^^AnnArbor, MI^48104^O |19890125^GHH&Good Health Hospital&L01||PMF88123453334|74160.2326@COMPUSERV.COM|B

2.10.5 Protocol for interpreting repeating fields in an update message

With repeating fields, the segment action codes are not relevant. Action codes cannot be applied to individual field repetitions, because they cannot be uniquely identified. Therefore, they must all be there, i.e., send a full list for each transaction. If the intent is to delete an element, it is left off the list. This is analogous to the snapshot mode for repeating segments and segment groups. If the intent is to delete the whole list, the field is transmitted once with a Null in the first component. In effect, the Sender must make a statement about what action the receiver is expected to take: omitting, or not populating, the field is not a clear signal according to field state definition as described in section 2.5.3.

At the same time, it is not incorrect to be precise about specific information that is to be deleted if the data type supports this capability. Note, however, that data types without components, e.g., IS or ST do not support this capability. There is no way to tie the Null value to an actual element instance in the persistent data store. See the example below.

Special consideration is warranted when implementing multiple interfaces. While the same processing rules (snapshot or update) can be applied to multiple systems and interfaces, desynchronization can occur if any one system is receiving similar information from multiple sources. Business rules and processes need to be considered in these cases to determine if there is a single authoritative source for the information (a "System of Record"), or if other business logic exists to resolve the possibility that information from the two (or more) sources are not in agreement.

Example: Repeating field of data type ID or IS: A patient is added to the Master Patient Index. The patient has two specific living conditions: "spouse dependent" and "medical supervision required". This is transmitted as:

MSH|^~\&||||||||ADT^A28^ADT_A05|1|P|2.6...<cr>

EVN|...<cr>

PID|||1234567^^^^MRN| <cr>

PV1|...<cr>

PD1|S~M|

Subsequently, the "medical supervision required" living condition is dropped.

MSH|^~\&||||||||ADT^A31^ADT_A31|1|P|2.6...<cr>

EVN|...<cr>

PID|||1234567^^^^MRN|<cr>

PV1|...<cr>

PD1|S||||||||||||||||||||||

The data type for PD1-1 is a data types without components. There is no way to tie the Null value to an actual element instance in the persistent data store. Therefore the following is ambiguous and not good practice.

MSH|^~\&||||||||ADT^A31^ADT_A31|1|P|2.6...<cr>

EVN|...<cr>

PID|||1234567^^^^MRN|<cr>

PV1|...<cr>

PD1|S~""||||||||||||||||||||||

2.11 LOCAL EXTENSION

The following section specifies where local extensions to a message and its constituent parts are allowed, where they are not, and where they are ill-advised. Inter-version compatibility rules must be followed plus there are certain restrictions and prohibitions outlined in the sections that follow. In general, basic structures should not be altered.

The reader is advised to review the Conformance mechanism defined in section 2B, "Conformance Using Message Profiles" before applying local extensions. Using the conformance mechanism may eliminate the need for local extension.

2.11.1 Messages

Messages may be locally extended as follows:

1. Users may develop local Z messages to cover areas not already covered by existing HL7 messages. These should be composed of HL7 segments where possible.

2. A local Z message may consist entirely of Z segments except that it must begin with a MSH segment.

3. A local Z Acknowledgement message must begin with an MSH segment followed by an MSA segment, an optional SFT segment and a conditional ERR segment.

4. Users may develop Z segments and add them to Z messages.

5. Users may develop Z segments and add them to HL7 messages. The trigger event may remain the same if the intent of the message has remained unchanged.

6. The practice of adding additional HL7 segments, like NTE, to existing HL7 messages locally is ill-advised. HL7 may move or change the segment in a future release; this will render the message unparsible.

2.11.2 Trigger events

Users may develop local Z trigger events for messages.

2.11.3 Segment groups

1. The practice of turning a single segment or segments into a segment group locally is not allowed within an HL7 event. It will have a negative impact on XML and any component-based encoding schemes. Note that HL7, on other hand, can do this.

2. A segment group may not be ungrouped locally.

Fox example, if there is an HL7 group as follows:

{

ABC

[DEF

[GHI]]

}

one cannot change it in a local implementation to be as follows:

{[ABC]}

[DEF]

[GHI]

Example 2:

If the original definition was:

GROUP1 ::= ABC, GROUP2?

GROUP2 ::= DEF, GHI?

and someone wished to constrain the segments in GROUP2 to be mandatory

(i.e., the HL7 grammar would look like:

{[

ABC

DEF

[GHI]

]}

Their message instance would need to still look like:

<GROUP1>

<ABC/>

<GROUP2>

<DEF/>

<GHI/>

</GROUP2>

</GROUP1>

It would be an error if they instead sent it as:

<GROUP1>

<ABC/>

<DEF/>

<GHI/>

</GROUP1>

1. A segment group can repeat locally. The 1st repetition needs to mean what it does in HL7

2. The practice of incorporating a Z segment into a segment group locally is allowed.

2.11.4 Segments

2.11.4.1 Local extension rules for segments

Users may not modify an existing segment, except as specified in section 2.8.2.

Locally defined fields may be defined for use in locally defined segments, although HL7 defined fields are a better choice when available. The practice of extending an HL7 segment with locally defined fields, while not prohibited, is ill-advised.

HL7 also recognizes that sites may have locally defined fields where the users believe the enhancement may be of interest to the HL7 community as a whole and are moving forward with a proposal to HL7.

2.11.4.2 Caveats for locally extending segments

Locally extending an HL7 segment with locally defined fields will likely cause conformance problems with the next release of the HL7 standard. There are, however, certain circumstances where HL7 has, itself, directed the membership to add Z fields as an interim measure between versions to accommodate regulatory agency requirements. These are fields that HL7 has reserved for official introduction in the next release.

If the local site intends to add a proposed field early, there is a risk that it may collide with another field when HL7 officially approves or rejects the proposed additions. Some sites have employed the practice of assigning a high sequence number locally, i.e., leaving a gap between the last official HL7 field and the proposed new field. The user-defined fields should be deleted or deprecated when HL7 officially approves or rejects the proposed additions so that the fields do not collide. It must be understood that the local implementation will have to adjust if a collision occurs and they want to conform.

2.11.5 Data types

The following rules apply for locally extending data types:

1. Locally defined data types may be defined for use in locally defined segment fields, although HL7 defined data types are a better choice when available.

2. Locally redefining existing data type components, e.g., changing a component from NM to ST, is prohibited.

3. Data types may be locally extended by adding new components at the end. This action creates a Z data type.

Note: The practice of extending an HL7 data type with locally defined components is particularly ill-advised and may cause conformance problems with the next release of the HL7 standard.

2.11.6 Tables

Rules for locally extending tables are the same as discussed in section 2.5.3.6, "Table":

1. Users may redefine suggested values in User-defined tables.

2. Local tables may be defined for Z fields.

3. Local tables may be assigned to HL7 fields with data type CWE.

2.12 CHAPTER FORMATS FOR DEFINING HL7 MESSAGES

Subsequent chapters of this document describe messages that are exchanged among applications in functionally-specific situations. Each chapter is organized as follows:

1. purpose. This is an overview describing the purpose of the chapter, general information and concepts.

2. trigger events and messages. There is a list of the trigger events.

3. message segments. The segments defined in a chapter are then listed in a functional order designed to maximize conceptual clarity.

4. examples. Complete messages are included.

5. implementation considerations. Special supplementary information is presented here. This includes issues that must be addressed in planning an implementation.

6. outstanding issues. Issues still under consideration or requiring consideration are listed here.

2.12.1 Message representation

For each trigger event the messages that are exchanged when the trigger event occurs are defined using the HL7 abstract message syntax as follows:

Each message is defined in special notation that lists the segment IDs in the order they would appear in the message. Braces, { . . . }, indicate one or more repetitions of the enclosed group of segments. Of course, the group may contain only a single segment. Brackets, [ . . . ], show that the enclosed group of segments is optional. If a group of segments is optional and may repeat it should be enclosed in brackets and braces, [{...}].

Note: [{...}] and {[...]} are equivalent.

Whenever braces or brackets enclose more than one segment ID a special stylistic convention is used to help the reader understand the hierarchy of repetition. For example, the first segment ID appears on the same line as the brace, two columns to the right. The subsequent segment IDs appear under the first. The closing brace appears on a line of its own in the same column as the opening brace. This convention is an optional convenience to the user. If there is conflict between its use and the braces that appear in a message schematic, the braces define the actual grouping of segments that is permitted.

A choice of one segment from a group of segments is indicated by using angle brackets to delimit the group and vertical bar delimiters between the several segments.

Example: The ORM^O01, as described in chapter 4 section 4.4.1, allows a choice of order detail segments. The choice would be represented as follows:

<OBR|RQD|RQ1|RXO|ODS|ODT>

Consider the hypothetical triggering event a widget report is requested. It might be served by the Widget Request (WRQ) and Widget Report (WRP) messages. These would be defined in the Widget chapter (say Chapter XX). The Widget Request message might consist of the following segments: Message Header (MSH), Software Segment (SFT), User Authentication Credentials (UAC), and Widget ID (WID). The Widget Report message might consist of the following segments: Message Header (MSH), Software Segment (SFT), Message acknowledgment (MSA), Error Segment (ERR) and one or more Widget Description (WDN) Segments each of which is followed by a single Widget Portion segment (WPN) followed by zero or more Widget Portion Detail (WPD) segments.

The ADD and DSC segments follow special rules or protocol as defined in section 2.10.2. They are not represented in the message grammar in the domain chapters as their presence is context sensitive.

2.12.2 HL7 abstract message syntax example

The schematic form for this hypothetical exchange of messages is shown in Figure 2-5:

Figure 2-5. Hypothetical schematic message

Trigger Event: WIDGET REPORT IS REQUESTED

WRP	Widget Report	Status	Chapter
MSH	Message Header		2
[{SFT}]	Software Segment		2
[UAC]	User Authentication Credential		2
MSA	Message Acknowledgment		2
[{ERR}]	Error Segment		2
{	---Widget begin
WDN	Widget Description		XX
WPN	Widget Portion		XX
}	---Widget end

The WID, WDN, WPN, and WPD segments would be defined by the widget committee in the widget chapter, as designated by the Arabic numeral XX in the right column. The MSH and MSA segments, although included in the widget messages, are defined in another chapter. They are incorporated by reference into the widget chapter by the chapter number XX.

On the other hand, the widget committee might decide that the WPN and WPD segments should appear in pairs, but the pairs are optional and can repeat. Then the schematic for the WRP message would be as shown in Figure 2-6.

Figure 2-6. WPN and WPD segments in pairs

If the widget committee determined that at least one pair of WPN and WPD segments must follow a WDN, then the notation would be as shown in Figure 2-7.

Figure 2-7. At least one pair of WPN and WPD

Acknowledgment messages may be defined on an application basis. However the simple general acknowledgment message (ACK) may be used where the application does not define a special message (application level acknowledgment) and in other cases as described in Section 2.9, "Message Processing Rules".

2.13.1 ACK - general acknowledgment

The simple general acknowledgment (ACK) can be used where the application does not define a special application level acknowledgment message or where there has been an error that precludes application processing. It is also used for accept level acknowledgments. The details are described in Section 2.9, "Message Processing Rules".

Note: For the general acknowledgment (ACK) message, the value of MSH-9-2-Trigger event is equal to the value of MSH-9-2-Trigger event in the message being acknowledged. The value of MSH-9-3-Message structure for the general acknowledgment message is always ACK.

2.14 MESSAGE CONTROL SEGMENTS

The following segments are necessary to support the functionality described in this chapter.

Note: The HL7 message construction rules define the standard HL7 encoding rules, creating variable length delimited messages from the segments defined below. Although only one set of encoding rules is defined as a standard in HL7 Version 2.3, other encoding rules are possible (but since they are non-standard, they may only used by a site-specific agreement).

The segments in this section are listed in alphabetical order. The following chart shows a summary of the segments listed by category.

Figure 2-8. HL7 message segments

The ADD segment is used to define the continuation of the prior segment in a continuation message. See Section 2.10.2, "Continuation messages and segments," for details.

HL7 Attribute Table - ADD - Addendum

2.14.1.1 ADD-1 Addendum Continuation Pointer (ST) 00066

Definition: This field is used to define the continuation of the prior segment in a continuation message. See section 2.10.2, "Continuation messages and segments" for details. When the ADD is sent after the segment being continued, it contains no fields. It is only a marker that the previous segment is being continued in a subsequent message. Thus fields 1-N are not present. The sequence designation, 1-N, means the remainder of the fields in the segment being continued. These remainder of the segment being continued fields are present only when the ADD is sent with a continuation message.

2.14.2 BHS - batch header segment

The BHS segment defines the start of a batch.

HL7 Attribute Table - BHS - Batch Header

2.14.2.1 BHS-1 Batch Field Separator (ST) 00081

Definition: This field contains the separator between the segment ID and the first real field, BHS-2-batch encoding characters. As such it serves as the separator and defines the character to be used as a separator for the rest of the message. Recommended value is | (ASCII 124).

2.14.2.2 BHS-2 Batch Encoding Characters (ST) 00082

Definition: This field contains the four characters in the following order: the component separator, repetition separator, escape characters, and subcomponent separator. Recommended values are ^~\& (ASCII 94, 126, 92, and 38, respectively). See Section 2.5.4, "Message ".

2.14.2.3 BHS-3 Batch Sending Application (HD) 00083

Definition: This field uniquely identifies the sending application among all other applications within the network enterprise. The network enterprise consists of all those applications that participate in the exchange of HL7 messages within the enterprise. Entirely site-defined.

2.14.2.4 BHS-4 Batch Sending Facility (HD) 00084

Definition: This field contains the address of one of several occurrences of the same application within the sending system. Absent other considerations, the Medicare Provider ID might be used with an appropriate sub-identifier in the second component. Entirely site-defined.

2.14.2.5 BHS-5 Batch Receiving Application (HD) 00085

Definition: This field uniquely identifies the receiving applications among all other applications within the network enterprise. The network enterprise consists of all those applications that participate in the exchange of HL7 messages within the enterprise. Entirely site-defined.

2.14.2.6 BHS-6 Batch Receiving Facility (HD) 00086

Definition: This field identifies the receiving application among multiple identical instances of the application running on behalf of different organizations. See comments BHS-4-batch sending facility. Entirely site-defined.

2.14.2.7 BHS-7 Batch Creation Date/Time (DTM) 00087

Definition: This field contains the date/time that the sending system created the message. If the time zone is specified, it will be used throughout the message as the default time zone.

2.14.2.8 BHS-8 Batch Security (ST) 00088

Definition: In some applications of HL7, this field is used to implement security features. Its use is not yet further specified.

2.14.2.9 BHS-9 Batch Name/ID/Type (ST) 00089

Definition: This field can be used by the application processing the batch. It can have extra components if needed.

Note: the text regarding "extra components" has been retained for backward compatibility, but it is not currently an accepted format for the ST data type.

2.14.2.10 BHS-10 Batch Comment (ST) 00090

Definition: This field is a comment field that is not further defined in the HL7 protocol.

2.14.2.11 BHS-11 Batch Control ID (ST) 00091

Definition: This field is used to uniquely identify a particular batch. It can be echoed back in BHS-12-reference batch control ID if an answering batch is needed.

2.14.2.12 BHS-12 Reference Batch Control ID (ST) 00092

Definition: This field contains the value of BHS-11-batch control ID when this batch was originally transmitted. Not present if this batch is being sent for the first time. See definition for BHS-11-batch control ID.

2.14.2.13 BHS-13 Batch Sending Network Address (HD) 02271

Definition: Identifier of the network location the message was transmitted from. Identified by an OID or text string (e.g., URI). The reader is referred to the "Report from the Joint W3C/IETF URI Planning Interest Group: Uniform Resource Identifiers (URIs), URLs, and Uniform Resource Names (URNs): Clarifications and Recommendations".

As with the Sending/Receiving Responsible Organization, the Sending Network Address provides a more detailed picture of the source of the message. This information is lower than the application layer, but is often useful/necessary for routing and identification purposes. This field should only be populated when the underlying communication protocol does not support identification of sending network locations.

The specific values and usage must be site negotiated

2.14.2.14 BHS-14 Batch Receiving Network Address (HD) 02272

Definition: Identifier of the network location the message was transmitted to. Identified by an OID or text string. (e.g., URL).

This is analogous with the Sending Network Address, however in the receiving role.

This field should only be populated when the underlying communication protocol does not support identification receiving network locations.

2.14.3 BTS - batch trailer segment

The BTS segment defines the end of a batch.

HL7 Attribute Table - BTS - Batch Trailer

2.14.3.1 BTS-1 Batch Message Count (ST) 00093

Definition: This field contains the count of the individual messages contained within the batch.

2.14.3.2 BTS-2 Batch Comment (ST) 00090

Definition: This field is a comment field that is not further defined in the HL7 protocol.

2.14.3.3 BTS-3 Batch Totals (NM) 00095

Definition: We encourage new users of this field to use the HL7 Version 2.3 data type of NM and to define it as "repeating." This field contains the batch total. If more than a single batch total exists, this field may be repeated.

Prior to v2.5 this field may have been defined as a CM data type for backward compatibility with HL7 Versions 2.2 and 2.1 with each total being carried as a separate component. Each component in this case is an NM data type.

2.14.4 DSC - continuation pointer segment

The DSC segment is used in the continuation protocol.

HL7 Attribute Table - DSC - Continuation Pointer

2.14.4.1 DSC-1 Continuation Pointer (ST) 00014

Definition: This field contains the continuation pointer. In an initial query, this field is not present. If the responder returns a value of null or not present, then there is no more data to fulfill any future continuation requests. For use with continuations of unsolicited messages, see chapter 5 and section 2.10.2, "Continuation messages and segments". Note that continuation protocols work with both display- and record-oriented messages.

2.14.4.2 DSC-2 Continuation Style (ID) 01354

Definition: Indicates whether this is a fragmented message (see Section 2.10.2, "Continuation messages and segments"), or if it is part of an interactive continuation message (see Section 5.6.3, "Interactive continuation of response messages").

Refer to HL7 Table 0398 - Continuation Style Code for valid values.

HL7 Table 0398 - Continuation style code

The ERR segment is used to add error comments to acknowledgment messages.

Use Cases:

Severity: A receiving application needs to communicate 2 "error or exception statements." One is an "error;" the other is a "warning". To accomplish this, an acknowledgement message with 2 ERR segments is sent. Upon receipt, the sending application can display both, including the appropriate severity, to the user.

Application Error Code: A receiving application generates an error that reports an application error code and returns this information in its response. This code in turn is used by helpdesk staff to pinpoint the exact cause of the error, or by the application to prompt an appropriate response from the user. (Ex. Deceased date must be greater than or equal to birth date).

Application Error Parameter: A receiving application encounters an error during processing of a transaction. In addition to an error code, the application provides an error parameter that gives greater detail as to the exact nature of the error. The receiving application looks up the message corresponding to the error code, substitutes in the parameter, and displays the resulting message to the user.

Diagnostic Information: While processing a transaction, a receiving application encounters an exception. When the exception is thrown, it provides a volume of detailed information relating to the error encountered. The receiving application captures the information and sends it in its response. The user reports the error to the help desk, and on request, faxes a copy of the diagnostic information to assist analyzing the problem.

User Message: A user executes an application function that generates a transaction that is sent to another application for further processing. During this processing, the receiving application encounters an error and, as part of the error handling routine, retrieves a User Message that it returns in its response. The originating application receives the error and displays it to the end user with the intent that the error condition can be resolved and the user can re-execute the function without error.

Inform Person Code: After submitting a dispense transaction, a response is returned to the user indicating that the patient may be abusing drugs. Given the sensitivity of this warning, the error is returned with an indicator stating that the patient should not be informed of the error with the implication that steps should be taken to rule out or confirm the warning.

Override Type: If a business rule states that a prescription on hold cannot be dispensed, an override type might be "Dispense Held Prescription" to allow the prescription to be dispensed in exception to the rule.

Override Reason Codes: A patient is given a prescription; however, before completing the prescription, the remaining pills are spoiled. The patient returns to their pharmacy and explains the situation to their pharmacist. The pharmacist decides to replace the spoiled drugs; however, when attempting to record the event, a message is returned indicating that the dispense would exceed the maximum amount prescribed. The pharmacist overrides the rule and specifies an Override Reason Code indicating a replacement of lost product.

Help Desk Contact: Help desk contact information is stored in a database. When an application error is encountered, the database is queried and the most current help desk contact information is returned in the error message. This is displayed to the user by the receiving application.

Better Error Location Information: Receiving system detects an error with the 3rd repetition of the ROL.4 (Role Person - XCN).16 (Name Context - CE).4(Alternate Identifier - IS). The application identifies the specific repetition and component when raising the error, simplifying diagnosis of the problem.

Support for multiple Error Locations: Two fields are marked as conditional, with the condition that one of the two must be specified. The sending application leaves both blank. The receiving application detects the problem, and sends back a single error indicating that one of the fields must be filled in. The ERR segment identifies both positions within the message that relate to the error.

HL7 Attribute Table - ERR - Error

2.14.5.1 ERR-1 Error Code and Location (ELD) 00024

Definition: This field identifies an erroneous segment in another message. Retained for backward compatibility only as of v 2.5; refer to ERR-2 and ERR-3 instead.

Refer to HL7 Table 0357 - Message Error Condition Codes for valid values.

2.14.5.2 ERR-2 Error Location (ERL) 01812

Definition: Identifies the location in a message related to the identified error, warning or message. If multiple repetitions are present, the error results from the values in a combination of places.

2.14.5.3 ERR-3 HL7 Error Code (CWE) 01813

Definition: Identifies the HL7 (communications) error code. Refer to HL7 Table 0357 - Message Error Condition Codes for valid values.

HL7 Table 0357 - Message error condition codes

Definition: Identifies the severity of an application error. Knowing if something is Error, Warning or Information is intrinsic to how an application handles the content. Refer to HL7 Table 0516 - Error severity for valid values. If ERR-3 has a value of "0", ERR-4 will have a value of "I".

Example: a Warning could be used to indicate that notes were present, but ignored because they could not be automatically processed, and therefore information could have been missed.

Example of Information: When submitting a claim, a payor might indicate remaining coverage under limit.

HL7 Table 0516 - Error severity

Definition: Application specific code identifying the specific error that occurred. Refer to User-Defined Table 0533 - Application Error Code for suggested values.

If the message associated with the code has parameters, it is recommended that the message be indicated in the format of the java.text.MessageFormat approach. This style provides information on the parameter type to allow numbers, dates and times to be formatted appropriately for the language.

User-defined Table 0533 - Application error code

Definition: Additional information to be used, together with the Application Error Code, to understand a particular error condition/warning/etc. This field can repeat to allow for up to 10 parameters.

Example: If the application error code specified in ERR.5 corresponded with the English message "The patient has a remaining deductible of {0, number, currency} for the period ending {1, date, medium}.", and the first two repetitions of ERR.6 were "250" and "20021231", then a receiving application in the U.S. would display the message as "The patient has a remaining deductible of $250.00 for the period ending Dec 31, 2002."

2.14.5.7 ERR-7 Diagnostic Information (TX) 01817

Definition: Information that may be used by help desk or other support personnel to diagnose a problem.

2.14.5.8 ERR-8 User Message (TX) 01818

Definition: The text message to be displayed to the application user.

Example:

|This program is having trouble communicating with another system. Please contact the help desk.|

This differs from the actual error code and may provide more diagnostic information.

2.14.5.9 ERR-9 Inform Person Indicator (IS) 01819

Definition: A code to indicate who (if anyone) should be informed of the error. This field may also be used to indicate that a particular person should NOT be informed of the error (e.g., Do not inform patient). Refer to User-defined table 0517- Inform Person Code for suggested values.

User-defined Table 0517 - Inform person code

Definition: Identifies what type of override can be used to override the specific error identified. Refer to User-defined table 0518 Override Type for suggested values.

User-defined Table 0518 - Override type

Definition: Provides a list of potential override codes that can be used to override enforcement of the application rule that generated the error. Refer to User-defined table 0519 - Override Reason for suggested values.

User-defined Table 0519 - Override reason

Definition: Lists phone, e-mail, fax, and other relevant numbers for helpdesk support related to the specified error.

2.14.6 FHS - file header segment

The FHS segment is used to head a file (group of batches) as defined in Section 2.10.3, "HL7 batch protocol".

HL7 Attribute Table - FHS - File Header

2.14.6.1 FHS-1 File Field Separator (ST) 00067

Definition: This field has the same definition as the corresponding field in the MSH segment.

2.14.6.2 FHS-2 File Encoding Characters (ST) 00068

Definition: This field has the same definition as the corresponding field in the MSH segment.

2.14.6.3 FHS-3 File Sending Application (HD) 00069

Definition: This field has the same definition as the corresponding field in the MSH segment.

2.14.6.4 FHS-4 File Sending Facility (HD) 00070

Definition: This field has the same definition as the corresponding field in the MSH segment.

2.14.6.5 FHS-5 File Receiving Application (HD) 00071

Definition: This field has the same definition as the corresponding field in the MSH segment.

2.14.6.6 FHS-6 File Receiving Facility (HD) 00072

Definition: This field has the same definition as the corresponding field in the MSH segment.

2.14.6.7 FHS-7 File Creation Date/Time (DTM) 00073

Definition: This field has the same definition as the corresponding field in the MSH segment.

2.14.6.8 FHS-8 File Security (ST) 00074

Definition: This field has the same definition as the corresponding field in the MSH segment.

2.14.6.9 FHS-9 File Name/ID (ST) 00075

Definition: This field can be used by the application processing file. Its use is not further specified.

2.14.6.10 FHS-10 File Header Comment (ST) 00076

Definition: This field contains the free text field, the use of which is not further specified.

2.14.6.11 FHS-11 File Control ID (ST) 00077

Definition: This field is used to identify a particular file uniquely. It can be echoed back in FHS-12-reference file control ID.

2.14.6.12 FHS-12 Reference File Control ID (ST) 00078

Definition: This field contains the value of FHS-11-file control ID when this file was originally transmitted. Not present if this file is being transmitted for the first time.

2.14.6.13 FHS-13 File Sending Network Address (HD) 02269

Definition: Identifier of the network location the message was transmitted from. Identified by an OID or text string (e.g., URI). The reader is referred to the "Report from the Joint W3C/IETF URI Planning Interest Group: Uniform Resource Identifiers (URIs), URLs, and Uniform Resource Names (URNs): Clarifications and Recommendations".

2.14.6.14 FHS-14 File Receiving Network Address (HD) 02270

Definition: Identifier of the network location the message was transmitted to. Identified by an OID or text string. (e.g., URL).

This is analogous with the Sending Network Address, however in the receiving role.

This field should only be populated when the underlying communication protocol does not support identification receiving network locations.

2.14.7 FTS - file trailer segment

The FTS segment defines the end of a file.

HL7 Attribute Table - FTS - File Trailer

2.14.7.1 FTS-1 File Batch Count (NM) 00079

Definition: This field contains the number of batches contained in this file.

2.14.7.2 FTS-2 File Trailer Comment (ST) 00080

Definition: The use of this free text field is not further specified.

2.14.8 MSA - message acknowledgment segment

The MSA segment contains information sent while acknowledging another message.

HL7 Attribute Table - MSA - Message Acknowledgment

2.14.8.1 MSA-1 Acknowledgment Code (ID) 00018

Definition: This field contains an acknowledgment code, see message processing rules. Refer to HL7 Table 0008 - Acknowledgment code for valid values.

HL7 Table 0008 - Acknowledgment code

Definition: This field contains the message control ID of the message sent by the sending system. It allows the sending system to associate this response with the message for which it is intended.

2.14.8.3 MSA-3 Text Message (ST) 00020

Definition: This optional field further describes an error condition. This text may be printed in error logs or presented to an end user.

The MSA-3 was deprecated as of v 2.4. The reader is referred to the ERR segment. The ERR segment allows for richer descriptions of the erroneous conditions.

2.14.8.4 MSA-4 Expected Sequence Number (NM) 00021

Definition: This optional numeric field is used in the sequence number protocol.

2.14.8.5 MSA-5 Delayed Acknowledgment Type 00022

Attention: The MSA-5 was deprecated as of v2.2 and the detail was withdrawn and removed from the standard as of v 2.5.

2.14.8.6 MSA-6 Error Condition (CE) 00023

Definition: This field allows the acknowledging system to use a user-defined error code to further specify AR or AE type acknowledgments.

The MSA-6 was deprecated as of v2.4. The reader is referred to the ERR segment. The ERR segment allows for richer descriptions of the erroneous conditions.

Editors Note: In the original 2.6 document the datatype for this field was changed to CWE. Since the MSA-6 was deprecated in V2.4, the datatype change from CE to CWE cannot be done.

Refer to HL7 Table 0357 - Message Error Condition Codes for valid values.

2.14.8.7 MSA-7 Message Waiting Number (NM) 01827

Definition: If present, indicates the number of messages the Acknowledging Application has waiting on a queue for the Requesting Application. These messages would then need to be retrieved via a query. This facilitates receiving applications that cannot receive unsolicited message (i.e., polling).

For example, if there are 3 low priority messages, 1 medium priority message and 1 high priority message, the message waiting number would be 5, because that is the total number of messages.

Use Case: An application that is playing a "requesting" role has limited network access to a centralized application playing a receiving role. When the requesting application contacts the acknowledging application with a regular update or query message, the acknowledging application replies with the appropriate response message, along with an indication that there are urgent messages waiting. The requesting application submits a query to retrieve the queued messages.

2.14.8.8 MSA-8 Message Waiting Priority (ID) 01828

Definition: If present, indicates that the Sending Application has messages waiting on a queue for the Receiving Application. These messages would then need to be retrieved via a query. This facilitates receiving applications that cannot receive unsolicited messages (i.e., polling). The code specified identifies how important the most important waiting message is (and may govern how soon the receiving application is required to poll for the message).

For example, if there are 3 low priority messages, 1 medium priority message and 1 high priority message, the message waiting priority would be 'high', because that is the highest priority of any message waiting.

With some applications, there is no guarantee that the sending application will be running. The receiving application is therefore unable to submit unsolicited messages. To mitigate this, a polling approach is used where the receiving application requests any queued messages when it is connected. To avoid the network overhead of continuous polling, the sending application needs a way to indicate that there are queued messages awaiting retrieval. It is also useful to provide an indication of the importance of those messages to indicate how quickly they should be retrieved.

Refer to HL7 Table 0520 - Message Waiting Priority for valid values.

HL7 Table 0520 - Message waiting priority

See MSA-7 above for Use Case.

2.14.9 MSH - message header segment

The MSH segment defines the intent, source, destination, and some specifics of the syntax of a message.

HL7 Attribute Table - MSH - Message Header

2.14.9.1 MSH-1 Field Separator (ST) 00001

Definition: This field contains the separator between the segment ID and the first real field, MSH-2-encoding characters. As such it serves as the separator and defines the character to be used as a separator for the rest of the message. Recommended value is | (ASCII 124).

2.14.9.2 MSH-2 Encoding Characters (ST) 00002

Definition: This field contains two to four characters in the following order: the component separator, repetition separator, escape character, and subcomponent separator. Best practice is to always include all four characters. Recommended values are ^~\& (ASCII 94, 126, 92, and 38, respectively). See Section 2.5.4, "Message delimiters'.

2.14.9.3 MSH-3 Sending Application (HD) 00003

Definition: This field uniquely identifies the sending application among all other applications within the network enterprise. The network enterprise consists of all those applications that participate in the exchange of HL7 messages within the enterprise. Entirely site-defined. User-defined Table 0361- Application is used as the user-defined table of values for the first component.

User-defined Table 0361 -Application

Note: By site agreement, implementers may continue to use User-defined Table 0300 - Namespace ID for the first component.

2.14.9.4 MSH-4 Sending Facility (HD) 00004

Definition: This field further describes the sending application, MSH-3-sending application. With the promotion of this field to an HD data type, the usage has been broadened to include not just the sending facility but other organizational entities such as a) the organizational entity responsible for sending application; b) the responsible unit; c) a product or vendor's identifier, etc. Entirely site-defined. User-defined Table 0362 - Facility is used as the HL7 identifier for the user-defined table of values for the first component.

User-defined Table 0362 -Facility

Note: By site agreement, implementers may continue to use User-defined Table 0300 - Namespace ID for the first component.

2.14.9.5 MSH-5 Receiving Application (HD) 00005

Definition: This field uniquely identifies the receiving application among all other applications within the network enterprise. The network enterprise consists of all those applications that participate in the exchange of HL7 messages within the enterprise. Entirely site-defined User-defined Table 0361- Application is used as the HL7 identifier for the user-defined table of values for the first component.

Note: By site agreement, implementers may continue to use User-defined Table 0300 - Namespace ID for the first component.

2.14.9.6 MSH-6 Receiving Facility (HD) 00006

Definition: This field identifies the receiving application among multiple identical instances of the application running on behalf of different organizations. User-defined Table 0362 - Facility is used as the HL7 identifier for the user-defined table of values for the first component. Entirely site-defined.

Note: By site agreement, implementers may continue to use User-defined Table 0300 - Namespace ID for the first component.

2.14.9.7 MSH-7 Date/Time of Message (DTM) 00007

Definition: This field contains the date/time that the sending system created the message. If the time zone is specified, it will be used throughout the message as the default time zone.

Note: This field was made required in version 2.4. Messages with versions prior to 2.4 are not required to value this field. This usage supports backward compatibility.

2.14.9.8 MSH-8 Security (ST) 00008

Definition: In some applications of HL7, this field is used to implement security features. Its use is not yet further specified.

2.14.9.9 MSH-9 Message Type (MSG) 00009

Definition: This field contains the message type, trigger event, and the message structure ID for the message.

Refer to HL7 Table 0076 - Message type for valid values for the message type code. This table contains values such as ACK, ADT, ORM, ORU etc.

Refer to HL7 Table 0003 - Event type for valid values for the trigger event. This table contains values like A01, O01, R01 etc.

Refer to HL7 Table 0354 - Message structure for valid values for the message structure. This table contains values such as ADT_A01, ORU_R01, SIU_S12, etc.

The receiving system uses this field to recognize the data segments, and possibly, the application to which to route this message. For certain queries, which may have more than a single response event type, the second component may, in the response message, vary to indicate the response event type. See the discussion of the display query variants in chapter 5.

2.14.9.10 MSH-10 Message Control ID (ST) 00010

Definition: This field contains a number or other identifier that uniquely identifies the message. The receiving system echoes this ID back to the sending system in the Message acknowledgment segment (MSA).

2.14.9.11 MSH-11 Processing ID (PT) 00011

Definition: This field is used to decide whether to process the message as defined in HL7 Application (level 7) Processing rules.

2.14.9.12 MSH-12 Version ID (VID) 00012

Definition: This field is matched by the receiving system to its own version to be sure the message will be interpreted correctly. Beginning with Version 2.3.1, it has two additional "internationalization" components, for use by HL7 international affiliates. The <internationalization code> is CE data type (using the ISO country codes where appropriate) which represents the HL7 affiliate. The <internal version ID> is used if the HL7 Affiliate has more than a single 'local' version associated with a single US version. The <international version ID> has a CE data type, since the table values vary for each HL7 Affiliate.

HL7 Table 0104 - Version ID

Definition: A non-null value in this field implies that the sequence number protocol is in use. This numeric field is incremented by one for each subsequent value.

2.14.9.14 MSH-14 Continuation Pointer (ST) 00014

Definition: This field is used to define continuations in application-specific ways.

Only the sender of a fragmented message values this field.

2.14.9.15 MSH-15 Accept Acknowledgment Type (ID) 00015

Definition: This field identifies the conditions under which accept acknowledgments are required to be returned in response to this message. Required for enhanced acknowledgment mode. Refer to HL7 Table 0155 - Accept/application acknowledgment conditions for valid values.

2.14.9.16 MSH-16 Application Acknowledgment Type (ID) 00016

Definition: This field contains the conditions under which application acknowledgments are required to be returned in response to this message. Required for enhanced acknowledgment mode.

The following table contains the possible values for MSH-15-accept acknowledgment type and MSH-16-application acknowledgment type:

HL7 Table 0155 - Accept/application acknowledgment conditions

Note: If MSH-15-accept acknowledgment type and MSH-16-application acknowledgment type are omitted (or are both null), the original acknowledgment mode rules are used.

2.14.9.17 MSH-17 Country Code (ID) 00017

Definition: This field contains the country of origin for the message. It will be used primarily to specify default elements, such as currency denominations. The values to be used are those of ISO 3166,.. The ISO 3166 table has three separate forms of the country code: HL7 specifies that the 3-character (alphabetic) form be used for the country code.

Refer to External Table 0399 - Country Code for the 3-character codes as defined by ISO 3166-1.

External Table 0399 - Country Code

Definition: This field contains the character set for the entire message. Refer to HL7 Table 0211 - Alternate character sets for valid values.

An HL7 message uses field MSH-18-character set to specify the character set(s) in use. Valid values for this field are specified in HL7 Table 0211, "Alternate Character Sets". MSH-18-character set may be left blank, or may contain one or more values delimited by the repetition separator. If the field is left blank, the character set in use is understood to be the 7-bit ASCII set, decimal 0 through decimal 127 (hex 00 through hex 7F). This default value may also be explicitly specified as ASCII.

More than one character set may be used in a message. The first occurrence, if supplied, of the MSH-18 must indicate the default encoding of the message. The second and subsequent occurrences of MSH-18-character set are used to specify additional character sets that are used.

The repetitions of this field to specify different character sets apply only to fields of the FT, ST and TX data types. See also section 2.7.2, "Escape sequences supporting multiple character sets".

Any encoding system, single-byte or multi-byte, may be specified as the default character encoding in MSH-18-character set. If the default encoding is other than 7-bit ASCII, sites shall document this usage in the dynamic conformance profile or other implementation agreement. This is particularly effective in promoting interoperability between nations belonging to different HL7 Affiliates, while limiting the amount of testing required to determine the encoding of a message.

By using built-in language functions for string and character manipulation, parsers and applications need not be concerned whether a single or double byte character set is in use, provided it is applied to the entire message. Using a built in function to extract the fourth CHARACTER will always yield the field separator character, regardless of coding set. On the other hand, if the parser looks at the fourth BYTE, it is then limited to single byte character sets, since the fourth byte would contain the low order 8 bits of the character S in a double-byte system.

Note: When describing encoding rules, this standard always speaks in terms of character position, not byte offset. Similarly, comparisons should be done on character values, not their byte equivalents. For this reason, delimiter characters should always have representation in the standard 7-bit ASCII character set, regardless of the actual character set being used, so that a search for the character CR (carriage return) can be performed.

HL7 Table 0211 - Alternate character sets

1. if the field is not valued, the default single-byte character set (ASCII ("ISO IR6")) should be assumed. No other character sets are allowed in the message.

2. if the field repeats, but the first element is NULL (i.e., present but unvalued), the single-byte ASCII ("ISO IR6") is assumed as the default character set.

3. elements in the remainder of the sequence (i.e., elements 2..n) are alternate character sets that may be used.

The reader is referred to the following references for background information on character sets and encodings:

• Unicode Technical Report #17 - Character Encoding Model (http://www.unicode.org/unicode/reports/tr17/)

• Extensible Markup Language (XML) 1.0 (Second Edition), Section F Autodetection of Character Encodings (http://www.w3.org/TR/REC-xml#sec-guessing)

2.14.9.18.1 Alphabetic Languages Other Than English

The first occurrence of MSH-18-character set may reference a character set other than 7-bit ASCII. Western alphabetic languages other than English are accommodated by the ISO 8859 series of character encodings. For example, if MSH-18-character set is valued 8859/1, the ISO character set commonly known as "8-bit ASCII" is in use in the message. This includes all values from decimal 0 through decimal 127 (hex 00 through hex 7F), plus an additional 128 values from decimal 128 through decimal 255 (hex 80 through hex FF). The latter values include the accented Latin letters used in common Western European languages, plus some symbolic values such as the paragraph mark (¶) and the trademark symbol (�).

Other ISO character sets in the 8859 series accommodate non-Latin character sets. For example, MSH-18-character set may be valued 8859/2 to specify the default character encoding in use in Eastern Europe, while 8859/6 indicates the use of the Arabic alphabet.

The ASCII and ISO character sets all allow the specification of any character in a single byte.

2.14.9.18.2 Non-Alphabetic Languages

HL7 Table 0211 includes values for languages that do not use alphabets. These include ideographic written languages, such as the Japanese Graphic Character Set which is specified as ISO IR87.

There are non-alphabetic encoding systems for which HL7 Table 0211 does not provide specific entries. One of these is the Traditional Chinese character set, CNS 11643, which is used in Taiwan. This character set can, however, be encoded using the Unicode Standard, which does have a value in HL7 0211.

The Unicode Standard (which is now coordinated with ISO 10646) permits the specification of multiple-byte characters in a much larger range than is available in a single-byte ASCII or ISO character set. Unicode Version 3.1 (http://www.unicode.org) includes almost 100,000 characters, including many Chinese, Japanese, and Korean ideographs. This is particularly valuable to implementers who need to encode messages in more than one character set, as for example to accommodate the use of both alphabetic and ideographic characters.

Non-alphabetic encoding systems do not restrict characters to a length of one byte. Unicode incorporates three encoding forms that allow for the use of multiple bytes to encode a message. The most flexible Unicode encoding form is UTF-8, which uses high-order bits to specify the number of bytes (from one to six) used to encode each character.

Interestingly, Unicode UTF-8 incorporates the 7-bit ASCII character set as single-byte codes. This means that a message encoded in 7-bit ASCII can be submitted to a destination using Unicode UTF- 8 with no modification.

2.14.9.19 MSH-19 Principal Language of Message (CWE) 00693

Definition: This field contains the principal language of the message. Codes come from ISO 639.

2.14.9.20 MSH-20 Alternate Character Set Handling Scheme (ID) 01317

Definition: When any alternative character sets are used (as specified in the second or later iterations of MSH-18 character sets), and if any special handling scheme is needed, this component is to specify the scheme used, according to HL7 Table 0356- Alternate character set handling scheme as defined below:

HL7 Table 0356 - Alternate character set handling scheme

Definition: Sites may use this field to assert adherence to, or reference, a message profile. Message profiles contain detailed explanations of grammar, syntax, and usage for a particular message or set of messages. See section 2B, "Conformance Using Message Profiles".

Repetition of this field allows more flexibility in creating and naming message profiles. Using repetition, this field can identify a set of message profiles that the message conforms to. For example, the first repetition could reference a vendor's message profile. The second could reference another compatible provider's profile or a later version of the first vendor profile.

As of v2.5, the HL7 message profile identifiers might be used for conformance claims and/or publish/subscribe systems. Refer to sections 2B.1.1"Message profile identifier" and 2.B.1.2, "Message profile publish/subscribe topics" for details of the message profile identifiers. Refer to sections 2.B.4.1, "Static definition identifier" and 2.B.4.2, "Static definition publish/subscribe topics" for details of the static definition identifiers.

Prior to v2.5, the field was called Conformance Statement ID. For backward compatibility, the Conformance Statement ID can be used here. Examples of the use of Conformance Statements appear in Chapter 5, "Query."

2.14.9.22 MSH-22 Sending Responsible Organization (XON) 01823

Definition: Business organization that originated and is accountable for the content of the message.

Currently, MSH provides fields to transmit both sending/receiving applications and facilities (MSH.3 - MSH.6). However, these levels of organization do not necessarily relate to or imply a legal entity such as a business organization. As such, multiple legal entities (organizations) may share a service bureau, with the same application and facility identifiers. Another level of detail is required to delineate the various organizations using the same service bureau.

Therefore, the Sending Responsible Organization field provides a complete picture from the application level to the overall business level. The Business Organization represents the legal entity responsible for the contents of the message.

Use Case #1: A centralized system responsible for recording and monitoring instances of communicable diseases enforces a stringent authentication protocol with external applications that have been certified to access its information base. In order to allow message exchange, the centralized system mandates that external applications must provide the identity of the business organization sending the message (Sending Responsible Organization), the organization it is sending the message to (Receiving Responsible Organization, in this case the "owner" of the communicable diseases system), the network address from which the message has originated (Sending Network Address), the network address the message is being transmitted to (Receiving Network Address). The organization responsible for protecting the information stored within the communicable disease system requires this authentication due to the sensitive nature of the information it contains.

2.14.9.23 MSH-23 Receiving Responsible Organization (XON) 01824

Definition: Business organization that is the intended receiver of the message and is accountable for acting on the data conveyed by the transaction.

This field has the same justification as the Sending Responsible Organization except in the role of the Receiving Responsible Organization. The receiving organization has the legal responsibility to act on the information in the message.

See MSH-22 above for Use Case.

2.14.9.24 MSH-24 Sending Network Address (HD) 01825

Definition: Identifier of the network location the message was transmitted from. Identified by an OID or text string (e.g., URI). The reader is referred to the "Report from the Joint W3C/IETF URI Planning Interest Group: Uniform Resource Identifiers (URIs), URLs, and Uniform Resource Names (URNs): Clarifications and Recommendations".

As with the Sending/Receiving Responsible Organization, the Sending Network Address provides a more detailed picture of the source of the message. This information is lower than the application layer, but is often useful/necessary for routing and identification purposes. This field should only be populated when the underlying communication protocol does not support identification of sending network locations.

An agreement about the specific values and usage must exist among messaging partners. Use Case:

Dr. Hippocrates works for the ''Good Health Clinic" (Sending facility) with a laptop running application XYZ (Sending App). He needs to talk to the provincial pharmacy system. He dials in and is assigned a network address. He then sends a message to the pharmacy system, which transmits a response back to him. Because the underlying network protocol does not have a place to communicate the sender and receiver network addresses, it therefore requires these addresses to be present in a known position in the payload.

There may be many doctors running application XYZ. In addition, the network address assigned to the laptop may change with each dial-in. This means there is not a 1..1 association between either the facility or the application and the network address.

MSH||RX|GHC|||||OMP^O09^OMP_O09||||||||||||||||05782|

Example 1: The Lone Tree Island satellite clinic transmits a notification of patient registration to its parent organization Community Health and Hospitals. The communication protocol does not support the identification of sending network location, so the sending network location is identified in the message by using its enterprise-wide network identifier "HNO2588".

MSH||Reg|Lone|||||ADT^A04^ADT_A04||||||||||||||||HN02588|

Example 2: The Stone Mountain satellite clinic transmits a notification of patient registration to its parent organization Community Health and Hospitals. The sending network location is identified by using its URI.

MSH||Reg|Stone|||||ADT^A04^ADT_A04|||||||||||||||| ^ftp://www.goodhealth.org/somearea/someapp^URI|

Example 3: The Three Rivers satellite clinic transmits a notification of patient registration to its parent organization Community Health and Hospitals. The sending network location is identified by using its Ipv4 address, port 5123 at node 25.152.27.69. The following example shows how to represent a port and DNS address using HD as the scheme

MSH||Reg|TRC||||| ADT^A04^ADT_A04||||||||||||||||5123^25.152.27.69^DNS|

Example 4: The Bayview satellite clinic transmits a notification of patient registration to its parent organization Community Health and Hospitals. The sending network location is identified by using "4086::132:2A57:3C28" its IPv6 address.

MSH||REG|BAY||||| ADT^A04^ADT_A04||||||||||||||||^4086::132:2A57:3C28^IPv6|

2.14.9.25 MSH-25 Receiving Network Address (HD) 01826

Definition: Identifier of the network location the message was transmitted to. Identified by an OID or text string (e.g., URL).

This is analogous with the Sending Network Address, however in the receiving role.

This field should only be populated when the underlying communication protocol does not support identification receiving network locations.

2.14.10 NTE - notes and comments segment

The NTE segment is defined here for inclusion in messages defined in other chapters. It is commonly used for sending notes and comments.

The technical committees define the meaning of the NTE segments within the context of the messages in their chapters. For each NTE, the description in the message attribute table should include an indication of the segment associated with the NTE, for example "Notes and Comments for the PID".

HL7 Attribute Table - NTE - Notes and Comments

2.14.10.1 NTE-1 Set ID - NTE (SI) 00096

Definition: This field may be used where multiple NTE segments are included in a message. Their numbering must be described in the application message definition.

2.14.10.2 NTE-2 Source of Comment (ID) 00097

Definition: This field is used when source of comment must be identified. This table may be extended locally during implementation. Refer to HL7 Table 0105 - Source of comment for valid values.

HL7 Table 0105 - Source of comment

Definition: This field contains the comment contained in the segment.

Note: As of v2.2, this field uses the FT rather than a TX data type. Since there is no difference between an FT data type without any embedded formatting commands, and a TX data type, this change is compatible with the previous version.

2.14.10.4 NTE-4 Comment Type (CWE) 01318

Definition: This field contains a value to identify the type of comment text being sent in the specific comment record. Refer to User-Defined Table 0364 - Comment Type for suggested values.

User-defined Table 0364 - Comment type

Note: A field already exists on the NTE record that identifies the Sources of Comment (e.g., ancillary, placer, other). However some applications need to support other types of comment text (e.g., instructions, reason, remarks, etc.). A separate NTE segment can be used for each type of comment (e.g., instructions are on one NTE and remarks on another NTE).

2.14.10.5 NTE-5 Entered By (XCN) 00224

Definition: This field contains the identity of the person who actually keyed the comment into the application. It provides an audit trail in case the comment is entered incorrectly and the ancillary department needs to clarify the comment. By local agreement, either the ID number or name component may be omitted.

2.14.10.6 NTE-6 Entered Date/Time (DTM) 00661

Definition: This field contains the actual date the comment was keyed into the application.

2.14.10.7 NTE-7 Effective Start Date (DTM) 01004

Definition: This field contains the date the comment becomes or became effective.

2.14.10.8 NTE-8 Expiration Date (DTM) 02185

Definition: This field contains the date the comment becomes or became non-effective.

2.14.11 OVR - override segment

Definition: This segment allows a sender to override specific receiving application's business rules to allow for processing of a message that would normally be rejected or ignored.

In many instances, business rules will be set as guidelines relative to patient care. In some instances it is in the patient's better interest to circumvent these guidelines. In other cases, business rules may exist to support normal process flow, but which may be bypassed or ignored under certain special circumstances. This segment is linked to the proposed ERR segment changes in that the first attempt to process a transaction that violates a business rule may result in an error that must be overridden. The ERR provides a mechanism to identify errors that may be overridden, as well as the allowed override codes.

Use case #1: A patient has received a prescription with a duration of 30 days and receives the full amount at their pharmacy. While at home the patient accidentally spills the container and spoils a significant proportion of the prescription. The patient returns to their pharmacy and explains the situation to the pharmacy technician. The technician consults with their supervising pharmacist. Knowing the patient, the pharmacist decides to override the business rule stating that the dispensed amount for a prescription may not exceed the prescribed amount. In recording the decision, the pharmacy technician specifies that the Override Type is a "Compassionate Refill" and that the Override Code, or reason for the override, is "Spoilage". The technician also provides Override Comments to provide an explanation of the situation for future reference. While recording the decision, the technician's user ID is automatically stored in an Override Recorded By field. The pharmacist's ID is stored in the Override Responsible Provider field.

Use case #2:A hospital wishes to submit an invoice to an insurer who is providing secondary coverage. The invoice is being submitted over a week after the service was performed, which is outside the insurer's normal accept time window. The insurer would normally reject the invoice. However, the submitter includes an Override Type of "late submission" as well as an Override Code indicating that the invoice is late due to delays with the primary payor. The secondary insurer examines the override reason and accepts the invoice.

Usage Note: The override segment should be included in messages adjacent to the segment(s) containing the information that would trigger the business rule(s) that needs to be overridden. The segment should be optional (you shouldn't always need to override business rules), and should be allowed to repeat in circumstances where there may be more than one business rule overridden at the same time. Committees may wish to provide suggested values for override types or codes for use with the OVR segment in different messages.

The following is an example of how the OVR segment might be used in a dispense message (RDS_O13):

MSH PID PV1 {ORC RXE {RXR} RXD {RXR} <RXC> <NTE> <FT1> <OVR>}

HL7 Attribute Table - OVR - Override Segment

2.14.11.1 OVR-1 Business Rule Override Type (CNE) 01829

Definition: Identifies what type of business rule override is being performed. Refer to User-defined Table 0518 - Override Type for suggested values. Given that an application provides end users with the ability to override business rules, there must be a way to communicate what business rule is being overridden.

2.14.11.2 OVR-2 Business Rule Override Code (CWE) 01830

Definition: Indicates the reason for the business rule override. Refer to User-Defined Table 0521 - Override Code for suggested values.

If users are allowed to override business rules in an application, the user will typically need to provide a reason why the rule is being overridden. The Override Code field in this segment will provide the mechanism to transmit a coded reason.

User-defined Table 0521 - Override code

Definition: Additional descriptive comments detailing the circumstances of the override.

When overriding a business rule there may be special circumstances that require a further explanation of the override action. The Override Comments field will allow users to provide more specific information in a free text format.

2.14.11.4 OVR-4 Override Entered By (XCN) 01832

Definition: Identifies the user entering the override in the system.

When a business rule is overridden, an application must be able to link the override with the user who made it in order to provide a complete audit history of the transaction, especially when there may be a risk associated with the override. In situations where the original message was submitted by batch, the overriding user may be different than the original author of the message.

2.14.11.5 OVR-5 Override Authorized By (XCN) 01833

Definition: Identifies the health services provider who accepts professional responsibility for overriding the business rule. This field should be left empty if the recording and responsible health care provider is the same as who entered the override.

In some cases, a business rule override may be entered by a data entry clerk on behalf of a health service provider who carries professional responsibility for the decision to override the rule. In order to represent this scenario, the segment must have a field identifying who is responsible for the override decision, in addition to the person recording the override.

2.14.12 SFT - software segment

Definition: This segment provides additional information about the software product(s) used as a Sending Application. The primary purpose of this segment is for diagnostic use. There may be additional uses per site-specific agreements.

Implementers are encouraged to use message profile identifiers (as found in 2.14.9.21, "MSH-21 Message Profile Identifier (EI) 01598") to control the behavior of the receiving application rather than relying on application or version information in the SFT segment.

For example, if software product A has versions 9 and 10 deployed in different Enterprise locations, the fact that they use different message types, segments, or fields should be reflected via their message profiles (see section 2B, "Conformance Using Message Profiles"). If there is an upgrade from version 10 to 10.1, this would be reflected in the SFT segment, but changes to the message contents should be reflected via a new/different conformance profile.

Use Case: An external application has been customized to communicate with a centralized patient drug history system. However, due to certain, known characteristics of the external software package, the centralized system must modify its behavior in order to process transactions correctly. In one example, the external application may have multiple versions in production. As such, the centralized application will need to know the name of the Software Vendor Organization, the Software Release Number, the Software Product Name, and the Software Binary ID so that it can correctly identify the software submitting the transaction and modify its behavior appropriately.

While preparing a transaction for submission to a centralized system the sending application specifies its Software Install Date and its configuration settings (Software Product Information). While processing the transaction, the centralized system encounters an error. Upon examination of the error, install date and configuration of the software that sent the message, helpdesk staff are able to determine the sending application has not been updated to reflect recent application changes.

Use Case: In circumstances where a message is manipulated or modified by multiple systems, a repetition of this segment may be appended by each system.

Example:

MSH

[{ SFT }]

...

HL7 Attribute Table - SFT - Software Segment

2.14.12.1 SFT-1 Software Vendor Organization (XON) 01834

Definition: Organization identification information for the software vendor that created this transaction. The purpose of this field, along with the remaining fields in this segment, is to provide a more complete picture of applications that are sending HL7 messages. The Software Vendor Organization field would allow the identification of the vendor who is responsible for maintaining the application.

2.14.12.2 SFT-2 Software Certified Version or Release Number (ST) 01835

Definition: Latest software version number of the sending system that has been compliance tested and accepted. Software Certified Version or Release Number helps to provide a complete picture of the application that is sending/receiving HL7 messages. Versions are important in identifying a specific 'release' of an application. In some situations, the receiving application validates the Software Certified Version or Release Number against a list of "certified" versions/releases of the particular software to determine if the sending application adheres to specific business rules required by the receiving application.

Alternatively, the software may perform different processing depending on the version of the sending software

2.14.12.3 SFT-3 Software Product Name (ST) 01836

Definition: The name of the software product that submitted the transaction. A key component in the identification of an application is its Software Product Name. This is a key piece of information in identifying an application.

2.14.12.4 SFT-4 Software Binary ID (ST) 01837

Definition: Issued by a vendor for each unique software version instance to distinguish between like versions of the same software e.g., a checksum.

Software Binary Ids are issued for each unique software version instance. As such, this information helps to differentiate between differing versions of the same software. Identical Primary IDs indicate that the software is identical at the binary level (configuration settings may differ).

2.14.12.5 SFT-5 Software Product Information (TX) 01838

Definition: Software identification information that can be supplied by a software vendor with their transaction. Might include configuration settings, etc.

This field would contain any additional information an application provides with the transaction it has submitted. This information could be used for diagnostic purposes and provides greater flexibility in identifying a piece of software. Possibilities include setup or configuration parameter information.

This field should not be sent unless performing diagnostics.

2.14.12.6 SFT-6 Software Install Date (DTM) 01839

Definition: Date the submitting software was installed at the sending site.

A Software Install Date on its own can often provide key information about the behavior of the application, and is necessary to provide a complete picture of the sending application.

2.14.13 UAC - User Authentication Credential Segment

Definition: This optional segment provides user authentication credentials, a Kerberos Service Ticket or SAML assertion, to be used by the receiving system to obtain user identification data. Refer to HL7 Table 0615 - User Authentication Credential Type Code. It is to be used in when the receiving application system requires the sending system to provide end-user identification for accountability or access control in interactive applications. Since user authentication implementations often limit the time period for validity of the session authentication credentials, this segment is not intended for use in non-interactive applications.

It is possible that various user authentication credential standards' data may be communicated. Kerberos and SAML are two such standards. A user authentication credential is an encapsulated data (ED type) element, as defined by standards, with no HL7-relevant structure.

Note: The UAC segment is defined for use within simple protocols, such as MLLP, that do not have user authentication semantics. Implementations that use WSDL/SOAP, or similar protocols, to envelope HL7 should employ the user authentication semantics and data structures available within the scope of those protocols rather than the UAC segment.

If the receiving system accepts the user credentials in the UAC segment, no specific acknowledgement is required. However, if the receiving system detects an error while processing the UAC segment, its acknowledgment message shall report it to the sender via an MSA and ERR segment pair:

The ERR-3 (error code) field value is 207 to signify an application error

The ERR-7 (diagnostic information) field reports the specific error. Examples of possible errors are:

User credentials expected but not provided

User credentials invalid

User credentials expired

User credentials from an unknown or untrusted source

User unknown

User not allowed to create or access data on the receiving system.

User not allowed to initiate a processing function on the receiving system.

When an MSA and ERR segment pair is reported to the sender, an application data response shall not occur. In such cases it is correct to assume that the sending application's user is not authorized to get the data.

The processing rules for the ERR segment are outside of HL7's scope.

HL7 Attribute Table - UAC - User Authentication Credential Segment

2.14.13.1 UAC-1 User Authentication Credential Type Code (CWE) 02267

Definition: This an identifier code for the type of user authentication credential.

HL7 Table 0615 - User Authentication Credential Type Code

Definition: This is user credential data as supplied by the sender's operating platform. The content and structure of this is defined by other standards and contain no HL7-relevant data.

2.15 DATA TYPES

HL7 Table 0440 - Data types

2.16.1 Message type table (0076)

HL7 Table 0076 - Message type

HL7 Table 0003 - Event type