IHE PIX Profile requires that the PIX Manager returns "AE" in MSA-1 and error code "204" in ERR-3 as an application error, when a patient identifier or a patient identification domain in a PIX request message is unknown (see 18.104.22.168.2.6, Cases 3 - 5).
HL7 Table 0357 (see 22.214.171.124, HL7 Standard 2.5) indicates, however, error 204 (Unknown Key Identifier) as "Rejection" in the Comment column of the table, which may imply "AR" (application rejection) code in MSA-1.
On the other hand, HL7 Standard 2.5 Section 126.96.36.199 descreibes the conditions of returning "AR":
- 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.
This seems aligned with the IHE requirement, to treat "unknown key identifier" as an application error rather than application rejection. To improve interoperbalibity, it is recommended that a PIX Manager implementation be prepared to handle both "AE" and "AR" code values in MSA-1 for error "204".