Increasingly it is challenging to understand when to use DiagnosticReport vs. DocumentReference.  The following is a suggestion based on a discussion in OO workgroup during the Wednesday Q4 session in Sydney to provide guidance that enables more consistent usage of the respective resources.

DiagnosticReport is response to a service request.  While the specific ServiceRequest may not be known, when it is reasonable to assume there was an original request it was in response to, then DiagnosticReport should be used.

DocumentReference is used to where the origins may not be as clear or reasonable to assume particularly when it involves non-FHIR artifacts, e.g., pdf-s, images, etc.  Generally, when the data intended to be referenced are represented using FHIR resource then such data should be directly referenced.  If the choice is made to do use DocumentReference, the rationale and value of injecting DocumentReference in the middle should be very clear, particularly why it could not be achieved otherwise.

Lastly, there are special cases where DocumentReference may be used for general documentation purposes in the absence of a reasonable alternative, e.g., anything else would work well (akin to the use of OBX in v2 beyond its core purpose).  An example would be the agreed to use of DocumentReference for Clinical Notes in FHIR US Core IG.