Background : The USCDI definition: Specimen Identifier Sequence of characters assigned by a laboratory for an individual specimen.   The Base FHIR Specimen resource definitions: Specimen.identifier Element Id Specimen.identifier Definition Id for specimen. Short Display External Identifier Note This is a business identifier, not a resource identifier (see  discussion ) Cardinality 0..* Type Identifier Summary true Specimen.accessionIdentifier Element Id Specimen.accessionIdentifier Definition The identifier assigned by the lab when accessioning specimen(s). This is not necessarily the same as the specimen identifier, depending on local lab procedures. Short Display Identifier assigned by the lab Cardinality   Decision: Mark both identifier and accessionIdentifier as  Must Support and  Although both are marked as Must Support, the server system is not required to support both, but  SHALL  support at least one of these elements. The client application  SHALL  support both elements.