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Overview

Search-type query on CodeSystem. This is the discovery call every FHIR client makes before invoking operations against a specific code system. HAPI FHIR’s RemoteTerminologyServiceValidationSupport.fetchCodeSystem() runs GET /CodeSystem?url=http://loinc.org to decide whether to route code validation through OMOPHub - if the response Bundle is non-empty, HAPI proceeds; if it’s empty, HAPI skips OMOPHub for that system. OMOPHub exposes per-vocabulary CodeSystem stubs with content: "not-present". The stubs are metadata-only; concept data is served via the $lookup, $validate-code, $expand, and $translate operations.

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Modes

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1. Search by canonical system URI

curl "https://fhir.omophub.com/fhir/r4/CodeSystem?url=http://loinc.org" \
  -H "Authorization: Bearer YOUR_API_KEY"
Returns a searchset Bundle with one entry - the LOINC stub.

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2. Search by the unified OMOP URL

curl "https://fhir.omophub.com/fhir/r4/CodeSystem?\
url=https://fhir-terminology.ohdsi.org" \
  -H "Authorization: Bearer YOUR_API_KEY"
Returns a searchset Bundle with one entry - the full OMOP omnibus CodeSystem (with all property definitions and the current vocabulary release metadata).

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3. List all supported CodeSystems

curl "https://fhir.omophub.com/fhir/r4/CodeSystem" \
  -H "Authorization: Bearer YOUR_API_KEY"
Returns a searchset Bundle with 21 entries - 20 per-vocabulary stubs plus the unified OMOP CodeSystem. Useful for building UI pickers and for one-shot capability discovery.

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4. Search by unknown URI

curl "https://fhir.omophub.com/fhir/r4/CodeSystem?url=http://example.com/unknown" \
  -H "Authorization: Bearer YOUR_API_KEY"
Returns an empty Bundle (total: 0), not a 404. This is the FHIR search convention - “no matches found”, not “endpoint not found”.

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Parameters

ParameterTypeRequiredDescription
urluriNoCanonical system URI to resolve. Omit to list all supported CodeSystems.

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Response (per-vocabulary stub)

{
  "resourceType": "Bundle",
  "type": "searchset",
  "total": 1,
  "entry": [
    {
      "fullUrl": "https://fhir.omophub.com/fhir/r4/CodeSystem/loinc",
      "resource": {
        "resourceType": "CodeSystem",
        "id": "loinc",
        "url": "http://loinc.org",
        "name": "LOINC",
        "title": "Logical Observation Identifiers Names and Codes",
        "status": "active",
        "experimental": false,
        "publisher": "Regenstrief Institute",
        "description": "Logical Observation Identifiers Names and Codes - served by OMOPHub. Use $lookup and $validate-code operations to query concepts; the full concept list is not served inline (content: not-present).",
        "content": "not-present"
      }
    }
  ]
}
The stub carries metadata only. content: "not-present" is the standard FHIR value for terminology servers that expose concepts via operations rather than inline enumeration. Clients should follow up with lookup](/api−reference/fhir−terminology/lookup),[lookup](/api-reference/fhir-terminology/lookup), [validate-code, or $subsumes using the same canonical URI (http://loinc.org) - the operation handlers dispatch on the FHIR system URI and route to the underlying OMOP vocabulary.

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Response (list-all mode)

Same Bundle shape as above, with 21 entries. Each stub has a distinct id (the short slug - loinc, snomed, rxnorm, etc.) and url (the canonical FHIR system URI). The final entry is the unified OMOP CodeSystem (https://fhir-terminology.ohdsi.org) with the release-specific id (e.g. omop-v20250827) and full property definitions.

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Stub-to-URI map

See the Supported Vocabularies table in the overview for the authoritative list of id ↔ url pairs.

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Errors

HTTPIssue CodeCause
401loginMissing or invalid API key
Unknown URIs return an empty Bundle, not an error.

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See Also