Defining Custom Terminology in FHIR

Not all concepts are documented in existing codes such as SNOMED CT or Loinc.

As such, most healthcare data projects reach a point where custom terminology is introduced.

This is bad for interoperability but often required for real-world projects.

FHIR projects are no different, but FHIR does provide clean ways of doing this that go some way to making custom terminology shareable.

Here are the steps to adding custom terminology to your FHIR project.

1. Create a CodeSystem resource in your FHIR server.
2. Give it a unique URI, preferably a URL that can be made public later if required.
3. Add each piece of custom terminology to the concept list in the CodeSystem, populating he following elements:
   – A unique code
   – A “display” text
   – A “definition” that provides a more formal description of the concept
   – Any other elements such as language and properties
4. Add your populated CodeSystem resource to your FHIR servers
5. Reference your new concepts in ValueSets as required

This should be your starting point. It “formalizes” your custom terminology — at least internally — and makes it easy to manage as your project matures.

More about the CodeSystem resource: https://hl7.org/fhir/R4/codesystem.html

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