Conformance Proposal on Original Mode
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.
Original Mode Should be Removed
In general, it is best for receiving systems to not make assumptions about information the sender knows. It is better to have the sender be explicit. Original mode allows the sender to not be explicit which requires the receiver to make an assumption.
The proposal is as follows:
- Remove all reference to Original Mode
- Remove the label "Enhanced Mode". Simply document one mode of response that is based on MSH-15 and MSH-16.
- Require senders to indicate both MSH-15 and MSH-16. (Move fields to R)