Question

When implementing an HL7 FHIR RESTful API, which specific mechanism must be used to represent a locally defined clinical observation that does not have a matching standard code in the core resource specification?

Accepted Answer

To represent a locally defined clinical observation that lacks a standard code in the FHIR specification, you must use the FHIR CodeSystem resource to define the concept locally and then reference it using a Coding object within the Observation resource. A CodeSystem acts as a formal definition of a set of codes, providing the system URL and the specific code value that identifies your local observation. Within the Observation resource, you represent this by populating the code element, which is a CodeableConcept type. The CodeableConcept contains a coding field where you specify the system, which is the unique URI of your local CodeSystem, and the code, which is the specific identifier for the local observation. If the local code does not have a formal FHIR CodeSystem resource defined, you must still provide the system URI that identifies your local context to ensure the code remains unambiguous. By using the code element populated with this local system and code pair, you allow receiving systems to identify and interpret the observation according to your specific local definitions while maintaining compatibility with the standard FHIR structure.

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When implementing an HL7 FHIR RESTful API, which specific mechanism must be used to represent a locally defined clinical observation that does not have a matching standard code in the core resource specification?