Expand a FHIR ValueSet using the terminology $expand operation to obtain a flat list of codes
domain: hl7.org · 6 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Identify the ValueSet by its canonical URL or server-assigned id; confirm the terminology server supports $expand via its CapabilityStatement
Send GET [base]/ValueSet/$expand?url=[canonical-url] or POST [base]/ValueSet/$expand with a Parameters body containing the url parameter
Optionally include filter to search within the expansion, count to limit results, and offset for pagination of large expansions
The server returns a ValueSet resource with an expansion element containing contains[] — each element has system, code, display, and optionally designation and property fields
If the expansion is incomplete (expansion.total > expansion.contains.length), use the offset parameter to page through remaining codes
Cache expansions locally with the expansion.timestamp and re-expand periodically when the underlying code system version may have changed
Known gotchas
Large ValueSets (e.g., all SNOMED CT concepts) may produce expansions with millions of codes; always specify a count limit and paginate rather than requesting an unbounded expansion
The expansion.total may not be populated by all terminology servers; do not rely on it for completeness checking — instead detect the end of pagination by receiving an empty contains array
ValueSet expansions are version-sensitive; always specify valueSetVersion and system version parameters when exact version reproducibility is required for regulatory or audit purposes
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