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Conformance Proposal on Original Mode

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HL7 v2 defines two acknowledgement modes: Original Mode and Enhanced Mode. The concept of Enhanced Mode has been in the HL7 v2 standard since 1996 when v2.2 was published. This means the concept of Enhanced Mode and the meaning of MSH-15 and MSH-16 has been established for two decades.

Enhanced Mode Includes Original Mode

The reason for the definition of "Original Mode" appears to be a strategy to provide backwards compatibility. In practice it is possible to use enhanced mode to specify original mode behavior.

Note from HL7 2.8.2 (page 31 of Chapter 2 Control)

   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).

Thus coding MSH=15 = NE and MSH-16 = AL is equivalent to not valuing either field to trigger original mode. Senders can choose to use Original Mode either by saying nothing (and leaving the receiver to assume Original Mode is being used) or to explicitly indicate it using Enhanced Mode rules.

Proposal: Original Mode Should be Removed

In general, it is best practice for sending systems to be explicit rather than depend on the receiver making assumptions. Original mode requires the receiver to make assumptions as to what the sender expects the receiver to do. Better to require the submitter to be explicit.

The proposal is to make the following changes in Chapter 2:

  • Add note indicating that the concept of "Original Mode vs Enhanced Mode" was in previously used for backwards compatibility to versions before HL7 v2.2 and this distinction of modes is no longer relevant.
  • Remove all mention and differentiation between Original Mode and Enhanced Mode in the rest of the document.
  • Indicate that response is to be based solely on MSH-15 and MSH-16 without defining the concept of "mode".
  • Require senders to indicate both MSH-15 and MSH-16. (Move fields to R)