HL7 V2.x Inter-Version Compatibility Guide

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A simple, practical and easy-to-follow Explanation of how to make Different Versions of HL7 V2.x Interoperate.

Are the Different Versions of HL7 V2.x Interoperable?

Are all the different V2.x Standards compatible or do all HL7 systems and interfaces in our Organisation have to be the same version?  Will V2.3.1 communicate with the new V2.7?  The question of compatibility and interoperability between different versions of HL7 is often asked.  Many systems designers are concerned that HL7-compliant systems using different versions of HL7 (e.g. V2.3.1, V2.4, V2.5, V2.5.1, V2.6 and the new V2.7) will not be compatible.  Quite the contrary is true: HL7 has been singularly successful in getting different versions of HL7 to interoperate.  HL7 has precise rules for compatibility between its versions.  These rules allow the sending and receiving of HL7 messages and converting their contents to data values with full backward compatibility between all 2.x versions of HL7.

As a result, messages created by systems using all HL7 standards of the V2.x series are fully compatible - if the business rules of sending and receiving systems are the same and the implementations have adhered to the HL7 version compatibility guidelines.  These guidelines are not well-known and even less well-understood.

In summary the rules are:

  1. New messages that may be part of a newer version of HL7 are ignored by any older versions.
  2. A newer version of HL7 may add new segments to a message, preferably at the end of the message.
  3. A newer version of HL7 may add new fields to the end of a segment, new components to the end of a field, new subcomponents to the end of a component, and a non-repeating field may be made repeating.
  4. New segments, fields, components, subcomponents and extra repetitions that are included in a newer version of HL7 are ignored by the older version.
  5. If a non-repeating field is made repeating in a newer version of HL7, the first instance of that repeating field must have the same meaning as the non-repeating field had in the older versions of HL7.
  6. The older version of HL7 ignores segments, fields, components, subcomponents and extra repetitions of a field that are present in the message from the newer version but were not expected.
  7. Treat segments that were expected but are not present as consisting entirely of fields that are not present.
  8. Data types may be enhanced in a newer version of HL7 if the leftmost parts of the data type has the same meaning as it had in the prior version of HL7 and new parts are added to the end of the data type structure.

In practice, correct implementation of the HL7 V2.x inter-version compatibility rules allows different versions of HL7 to exchange data without any significant problems.  Of course, new functionality added by a newer version may not be supported by an older version.  In this case the business rules of interoperating must be agreed on by sender and receiver.

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For more information, refer to section 2.11.2 in V2.4 of the HL7 Standard!

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