There is no obvious way for a Procedure or DiagnosticReport to reference another relevant Procedure/DiagnosticReport. This would be helpful for referring physicians who may want to navigate from a current radiology report which discusses a mass of some size to the prior study a month ago where the mass was another size. I can envision a number of situations where a current report would want to reference a "prior," "comparison", "relevant," or "baseline" report. This is particularly useful if the patient has a large number of Procedures/reports, and it isn't immediately obvious which would be the relevant prior.

I don't think that linking at the Observation is the correct approach (although doing that as well could be useful), since (a) the compared content is often at the level of the report, not the individual observation, and (b) some practitioners (particularly outside the institution) would only have access to the report, not the individual observations.

I don't think that basedOn really fulfills the need here, and there isn't a standard way of flagging what the nature of the relationship is with basedOn anyhow.

I suggest addition of a standard extension, or addition of new elements to DiagnosticReport, which allows expressing a related DiagnosticReport or Procedure, and the type of the relation (likely via a small controlled vocabulary). It isn't clear to me whether we only need to be concerned with DiagnosticReports, or whether Procedures and ServiceRequests (or other resources) can have similar relationships to other Procedure/ServiceRequests.