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Terminology server

In short

A terminology server is the system that holds clinical code catalogues, such as SNOMED CT, LOINC or ICD-10, and answers questions about them: whether a code exists, what it means, which list it belongs to or how it maps to another catalogue. It is the piece that lets different systems use codes the same way. Several countries run a national one.

Type
Technology
Steward
No single steward: general concept or generic technology
Reviewed
October 25, 2026

Explanation

What it is

A terminology server is a software component that manages code systems (SNOMED CT, LOINC, ICD-10, local catalogues), value sets (the lists of codes allowed for a field) and maps between systems, and offers them as a service to other applications. It is a generic technology: it has no single owner, and it is run by organizations ranging from national agencies to standards communities.

The FHIR interface

HL7 FHIR defines what a server must offer to be called a "FHIR Terminology Service": JSON and XML formats, read and search on the CodeSystem, ValueSet and ConceptMap resources, and these operations:

OperationWhat it does
$lookupGet the name and properties of a code
$validate-codeCheck whether a code exists or belongs to a value set
$subsumesFind out whether a concept is a subtype of another
$expandList the codes in a value set, with filtering and paging
$translateTranslate a code from one system to another using a map

The server also declares which terminologies and versions it supports with a TerminologyCapabilities resource.

Common misconceptions

  • A terminology server is not a terminology. Without loaded, up-to-date and licensed content it is useless.
  • Content licensing is separate. SNOMED CT, for example, requires a licence, and national servers control access accordingly.
  • Versions matter. A code that is valid in one edition can be inactive in another, and validating against the wrong version gives misleading results.
  • A map between systems (for example, from SNOMED CT to ICD-10) is not always one-to-one. $translate can return several candidates, and someone must validate which one applies.
  • Centralizing is a governance decision. As NHS England points out, a national server spares every system from building and managing its own terminology database, but it requires publication, licensing and availability rules.

Usage examples

  1. Australia's National Clinical Terminology Service runs a national terminology server with FHIR endpoints and publishes SNOMED CT-AU every month.

  2. The NHS England Terminology Server offers SNOMED CT, dm+d, ICD-10 and OPCS through FHIR APIs. It is provided free of charge, and the use of each content is subject to its individual licence agreement.

  3. The FHIR implementation guide publisher validates each guide's codes against the community terminology server, tx.fhir.org.

  4. The US National Library of Medicine (NLM) offers the FHIR Terminology Service for VSAC Resources, an API to query and expand VSAC value sets, which requires a free UMLS licence.

  5. WHO has added a pre-release FHIR R5 service to the ICD-API for querying ICD-11 and ICF, in line with the IHE SVCM profile.

  6. A health record form calls $expand with a text filter to show only the allowed diagnoses that match what the clinician is typing.

  7. A rules engine uses $subsumes to check whether the recorded diagnosis is a subtype of viral hepatitis in SNOMED CT before firing an alert.

  8. A laboratory translates its internal test codes to LOINC with $translate and a ConceptMap maintained by the terminology team.

Relation to other terms

Relies on2

  • Value setStrong

    Expanding and validating value sets ($expand, $validate-code) is one of its core functions.

  • FHIRMedium

    FHIR defines a standard terminology service interface, but a terminology server can offer other interfaces.

Depend on this one3

  • OpenHIEStrong

    The terminology service is one of its registry services and has its own workflows, such as expanding value sets or translating codes.

  • DPI-HMedium

    The framework groups terminology services with the canonical registries, to map codes during exchange.

  • HIEMedium

    OpenHIE includes a terminology service to normalize which services were provided; not every HIE centralizes it.

Sources

  1. HL7 FHIR — Using Codes: Terminology Servicehl7.org
  2. Australian Digital Health Agency — National Clinical Terminology Service (NCTS)healthterminologies.gov.au
  3. NHS England — Terminology Serverdigital.nhs.uk
  4. NLM — FHIR Terminology Service for VSAC Resourcesnlm.nih.gov
  5. WHO — ICD-API FHIR support (pre-release)icd.who.int
  6. HL7 — Terminology server report of a published FHIR guide (tx.fhir.org)hl7.org
Health interoperability glossary