The philosophy of HL7V2.x is based on the assumption that a real event that occurs in a healthcare setting creates data that needs to be exchanged between different systems of that setting. This is essentially to eliminate duplication of data collection from the patients and to automate administrative process saving time and money.
The real world events e.g. patient admitted or Lab orders raised are represented by trigger events and messages are specified for each trigger event. The structure of an HL7v2.x message is shown in Figure 1 below
Segments - Version 2.X messages are designed around message segments indicated by a three letter segment id e.g. PID – Patient Identification; ORC- Common Order etc. Each message is composed of a group of segments in a defined sequence. Segments of message many be mandatory or optional and may repeat .A segment is a delimited line of data that contains data relevant to one specific use in the message. For example, patient identification information is in PID segment while patient allergies are defined in AL1 segment. Groups can be thought of as simply a collection of segments or other groups.
The HL7V2.x standard specification defines the standards segments. However the messages can also contain segments that are not part of the standard. These segments are locally-defined segments, called “Z-Segments” as their Segment ID code begins with a “Z”. These are allowed to add flexibility to the standard and usually appear at the end of the relevant segment group.
Each segment is a logical grouping of data fields. then composed of data fields. Each data field can have one or more components and each component may have sub-components.
HL7 1.0 originated in 1987 as an open standard for vendors for interfacing different clinical systems within an organization. It was defined in a period of six months with initial set of Orders and ADT messages. They were not widely implemented but laid a foundation for defining standard healthcare interfaces. HL7 v2.0 which was demonstrated at 1989 HIMSS convention introduced the concept of triggers, added additional detail to the Message header, and expanded the set of messages to include billing. HL7V2.0 was also not widely implemented. Figure 2 below shows a timeline of HL7 Version 2 releases.
Figure 2: Version 2 Timeline
HL7 Version 2.2 - HL7 Version 2.2 is primarily centered around cleaning up and clarifying version 2.1 specifications The notable changes to version 2.2 among others is that Segments were added to existing events for e.g. Merge Segment was added to the Transfer events (A06 & A07); Next of Kin Segment was added to the Patient Query (A19) event. Some additional fields were added to segments for e.g. additional fields to PID segment to handle newborn baby information and Next of Kin was made more generic to handle any person associated with the patient. New messages to handle Pharmacy, Diet and supply orders were added. Apart from adding new data types the HL7 acknowledgment paradigm has been extended to distinguish both accept and application acknowledgments, as well the conditions under which each is required.
HL7 Version 2.3 - Version 2.3 offers new data types, especially the 'x'-tended versions of PN, CN, etc. Enhanced query functionality and clear definitions of tables for e.g. differentiating ID/IS data types. In Financial Management Additional messages were added and message constructs were extended to allow greater specificity in the intent of the message, and to report additional information.
HL7 V2.x Message Construction
Message Encoding Rules: Version 2.x messages have some defined special characters that delineate data fields which make up the message. These special characters are the segment terminator, the field separator, the component separator, the sub-component separator, and the repetition separator. Table 1 below shows the definition of these separators
Table 1: Version 2 Separators
Message Construction Rules: A Version 2.x message consists of segments. Message segments are constructed in the order defined below
1. The first three characters in the message segment that will be inserted into the message is the segment ID code, e.g. - PID – Patient Identification Details.
2. Each segments consists of different fields and each of those data fields that make up a segments are inserted into the segment in sequence:
2.1 A field separator ¦XYZ¦ is placed at the beginning and the end of the field. If there is no value present in the field then no other characters are required and a ¦¦ is used.
2.2 If the value is present but equivalent to null, two consecutive quotation marks are placed in the field ¦""¦.
2.3 If a field is defined to have components (combination of meaningful data fields), the following rules apply:
- Components are separated by the component separator.
- Components that are present but null are represented by two consecutive quotation marks
- Components with no value require a component delimiter but do not require characters in the component. For example: ¦ABC^^DEF¦
- Components at the end of a data field with no value do not have to be represented by a component delimiter. For example: ¦ABC^DEF^^¦ = ¦ABC^DEF¦
- When a component is itself a data type that contains components, its delimiters are demoted by one. For example, the component delimiters for the composite field ¦ABC^DEF¦ are demoted by one to ¦JKL^ABC&DEF¦
2.4 If component has sub-components, the following rules apply:
- Sub-components are separated by the sub-component separator.
- Sub-components that are present but null are represented by two consecutive quotation marks.
- Sub-components that are not present require place holder separator but no characters in the sub-component.
- Sub-components at the end of a data field with no value do not have to be represented by a sub-component delimiter.
2.5 If the field definition permits repetition, the repetition separator is used only if more than one occurrence is transmitted. The repetition separator is placed between occurrences.
2.6 End each segment with an ASCII Carriage Return character - ASCII(13), HEX(0D).
3. Steps 1 and 2 are repeated until all segments have been generated.
4. The receiving system will ignore segments, fields, components, and extra repetitions that are present but not expected. The receiving system will treat segments that are expected but not present as consisting entirely of fields that are “not present”, and it will treat fields and components that are expected but not included in a segment as “not present”.
Data Types: Data types define the structure of the fields. Data type specification is an important tool for simplifying the complexity of the HL7 standard, and is critical for understanding the data contents of an HL7 field.
Version 2 supports the following data types
- Primitive – consisting of one component e.g. ST(String)
- Simple – multiple components, no ‘type’ code e.g. CE (Coded Element)
- Complex – multiple components with a ‘type’ code e.g. XAD(Extended Address)
The application of data types to the fields can be gauged by the following examples.
The format of CE as per version 2 specification is
CE :< identifier (ST)> ^ <text (ST)> ^ <name of coding system (IS)> ^ <alternate identifier (ST)> ^ <alternate text (ST)> ^ <name of alternate coding system (IS)>
11289-6 is the identifier; Body Temperature is the text and LN (LONIC) is the name of the coding system.
The format of XAD as per version 2 specification is
XAD : <street address (ST)> ^ <other designation (ST)> ^ <city (ST)> ^ <state or province (ST)> ^ <zip or postal code(ST)> ^ <country (ID)> ^ < address type (ID)> ^ <other geographic designation (ST)>^ <county/parish code
(IS)> ^ <census tract (IS)>
e.g.: ¦23 Sussex Place ^Flat 3 ^ Slough ^ Berkshire^SL11NH^UK^P^^BERK^¦
23 Sussex Place corresponds to street address ; Flat 3 for other designation; Slough for city; Berkshire for state;SL11NH for postcode; UK for country and P which stands for permanent.
HL7V2.x Message Anatomy
Imagine the following scenario
- Patient Andy Smith Carol was admitted on July 22, 2006 at 11:23 a.m. by Doctor John Carol (#C145678) for an orthopedic procedure (ORT).
- He has been assigned to room 202, bed 10 on nursing unit 1000.
- The Patient Lives at 23, Sussex Place, Slough, Berkshire, SL11NH and was born on December 23rd 1961.
- The hospital the Mary Green Maternity – MGM has PAS System - ADT1 and a laboratory system PATHLAB.
The admission of the patient corresponds to a trigger for Admit patients and an event A01 is intended for admitted patients. This will be sent from PAS system to Lab system in real time as soon as the admission takes place. According to HL7 V2.3.1 message specifications a portion of message structure of A01 shown in Table 2
Table 2: A01 Message Structure
Download the complete A01 HL7 V2.3 message specifications from HL7 website (http://www.hl7.org/)
The most important fields in the message segment (MSH) header are shown below in Table 3.
Table 3: Relevant MSH Fields
The message segment MSH shown below sent in the message based on the scenario above shows what has been sent in the following scenario with the data items from the PAS database populating these fields.
The other important message segment PID in A01 whose important fields are shown in Table 4. The complete list of fields is available from v2.3.1 Message pack on HL7 Website.
Table4: Relevant PID fields
The message segment PID shown below sent in the message based on the scenario above shows what has been sent in the following scenario with the data items from the PAS database populating these fields.
Messaging Communication: The most commonly used mechanism for sending HL7 Version 2.x messages is Lower Layer Transport protocol (LLP). The messages are sent over TCP/IP over the network.
The protocol is very simple to implement using the implementation setting sockets. The protocol requires that that each message should be preceded with the character 0x0B (11) and followed with the characters 0x1C (28) and 0x0D (13). This is required for communications code to be able to recognize the start and the end of each message
Finally each segment is terminated by a 0x0D (13) character. This is mandated by the standard, but often HL7 log data can be received via ftp or email where the segment separators have been transformed into 0x0A characters, etc
Current Status of HL7V2
Some countries such as Germany have legislation that stipulates the use of V2.X, and V2.4 which is a ANSI standard from 2004 is now being proposed as an ISO standard, making it a truly international standard though it have been developed for American market.
The Version 2.x standards are widely used in different countries in the world, in particular in acute healthcare settings and in some countries even in primary care. The HL7 versions 2.1 to 2.5 which have been published are in wide use and the work is underway on V2.6, which is expected to be published late 2005/early 2006.
Version 2.5 – HL7 Version 2.5 is the last published standard in the HL7 Version 2.x series. Version 2.5 contains new messages and updates to HL7 Version 2.4. Version 2.5 is supposed to be broader in scope that Canadian Health Information Technology Trade Association (CHITTA) have issued a position paper to Canada Health Infoway urging the agency to reconsider its support of Health Level 7 (HL7) Version 3 in view of the supposed unavailability of Version 3 normalised specifications (Ref: Section xxx) and asked it to endorse HL7V2.5 which provides greater interoperability. HL7 Version 2.5 introduced a number of new events, segments and messages and expanded the Control section. Version 2.5 is more consistent and supports more functionality than any of the previous versions. The significant modifications to version 2.5 are
- Better and clearer documentation of the data types
- A definition of a message profile methodology
- Better support for Radiology/Imaging by means of a new segment and a new order message
- Support for orders related to blood products
- New update message for diagnosis/procedures
- New specification of claims and reimbursement messages
Version 2.6 - HL7 Version 2.6 completed the first ballot cycle in September 2004 and is nearing completion and will be released in 2006. V2.6 will include enhancements made to allow the communication of Electronic Health Record (EHR) information which is sighted as one of the biggest deficiency of version2.x. Version 2.6 is an upgrade to HL7 Version 2.5 and it includes new segments, fields, components and subcomponents necessary to apply either a documented regulatory requirement or a harmonization requirement with HL7 Version 2 material created by other committees. The main changes to version 2.6 from version 2.5 are
- Addition of general new segment -UAC "User Authentication Credential"
- Better management of access to sensitive patient information by adding Access Restrictions (ARV) segment.
- New message type to support the US National Animal Health Laboratory Network (NAHLN)
- Deprecation of data types such as CNN,LA1, LA2 and NDL data types
- Addition of components to the XAD and XTM data types.
- "TS" Timestamp data type is being replaced by the DTM "Date/Time" data type.
- CE "Coded Element" data type is being replaced by either the CNE "Coded with No Exceptions" or the CWE "Coded with Exceptions" data types
- Code tables defined by external standards organizations will not be considered as HL7 tables but an HL7 number will be assigned.
- Many new fields have been added to the segments in the Financial Management
- “Observation Reporting” chapter has added support for referral and shared care
- Personnel Management chapter contains new attributes to the STF and ROL segments.
- New chapter Materials Management with messages for communicating various events related to the appointment scheduling for services and resources
XML encoding :The German HL7 group defined a comprehensive database of the HL7 standard to allow consistency checks of items and to support the application of the standard by the user when they realized that the different chapters in the published standard have been developed by different groups and there are no distinct rules or guidelines for the development of various parts of the standard.
The database is a Microsoft Access database and contains the official definitions for events, messages, segments, fields, data types, components, tables and values. An XML representation of version 2.x standard is algorithmically derived directly from this database. The algorithm consists of SQL queries to extract tables which are then exported to ASCII delimited files. Perl scripts are applied to these ASCII files to generate XML DTD’s.
Two sets of DTD’s have been provided;
A single DTD (hl7_v231.dtd) that contains all HL7V2.3.1 definitions. This file is broken up into four separate DTD definitions – Message.dtd; segments.dtd; fields.dtd datatypes.dtd.
One DTD for each message structure. Each imports the same datatype DTD referenced by hl7_v231.dtd, but is otherwise self contained with all message, segment and field declarations needed for the message structure.
The XML representation represents HL7 message structures as XML elements. Message structures contain segments, also represented as XML elements. Segments contain fields, again representedas XML elements. A field's data type is stored as a fixed attribute in the field's attribute list, while a field'scontent model contains the data type components. Other fixed attributes are used to expand abbreviations and indicate HL7 Table value restrictions
A simple version 2.x representation is shown below
The representation of the above message using XML encoding rules is shown below.
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