SNOMED CT Implementation Guide for the LOINC Ontology

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When working with the LOINC Ontology, leveraging a FHIR terminology server can simplify access and management. The server allows you to handle the content as a specific CodeSystem version, providing access to all FHIR Terminology operations.

LOINC Extension FHIR URIsing

The SNOMED International URI standard specifies that identify unversioned editions (i.e. editions) and versioned editions (i.e. versions) take the following respective forms:

These URIs apply to the 'version' element of codes and the 'URL' parameter in FHIR Operations. The 'system" element value is always 'http://snomed.info/sct' for all SNOMED editions. For more information on the URI Specification, please refer to the URI Standard document.

Examples

The following table shows some examples of URIs for editions and versions relevant when working with the LOINC Ontology.

ResourceURI

SNOMED CT International Edition

http://snomed.info/sct/900000000000207008

SNOMED CT International Edition, 20250101

http://snomed.info/sct/900000000000207008/version/20250101

SNOMED CT-LOINC

http://snomed.info/sct/11010000107

SNOMED CT-LOINC, 31 March 2025

http://snomed.info/sct/11010000107/version/20250531

FHIR Operations

The following table lists a few key requests relevant for retrieving content from the LOINC Ontology using the FHIR Terminology services API.

Service Name and Status

Input

Output

$lookup

A conceptId and a CodeSystem url
  • Concept metadata, including status, designations, etc.

$expand

Get all members of the reference set

REQUIRED

  • A reference set specified by its refsetId, in this case the identifiers for the set of Orderable and Observable LOINC Terms:

  • A list of concept or description IDs
  • Option to include additional information about each concept or description

$validate-code

Test if a concept or description is a member of a specified reference set

REQUIRED

  • A reference set specified by its refsetId

  • A candidate concept.id
  • If the candidate concept is a member of the reference set: TRUE 
  • Otherwise: FALSE

Please refer to the SNOMED CT Terminology Services Guide for a comprehensive guidance on SNOMED CT enabled terminology services. 

Example FHIR Requests

These examples use the concept   635111010000100 |LOINC Orderable Reference Set| as the refsetId.

Service Name

API Call

Result
Get concept metadata
GET fhir]/ValueSet/$expand
?url=http://snomed.info/sct?fhir_vs=refset/[refsetId]
&code=537131010000109
Returns a JSON representation of concept metadata
Get all members of the reference set
GET [fhir]/ValueSet/$expand
?url=http://snomed.info/sct?fhir_vs=refset/[refsetId]
&count=10

for example

GET [fhir]/ValueSet/$expand
?url=http%3A%2F%2Fsnomed.info%2Fsct%3Ffhir_vs%3Drefset%2F635111010000100&count=10

An alternative solution is to use the expression constraint language, as shown here:
GET [fhir]/ValueSet/$expand
?url=http%3A%2F%2Fsnomed.info%2Fsct%3Ffhir_vs%3Decl%2F%5E%5B635111010000100%5D
&count=10
GET [fhir]/ValueSet/$expand
?url=http%3A%2F%2Fsnomed.info%2Fsct%3Ffhir_vs%3Decl%2F%5E635111010000100&count=10

Returns a JSON representation of data about each of the reference set member.

The data returned for each concept includes:

  • code:  the conceptId of the reference set member
  • display:  the preferred term for the reference set member

Also returns the  total  number of reference set members

As some reference sets have very large numbers of children, this service is paged. Requests parameters include:

  • count  to restrict the number of members returned.
  • offset  to specify the start in the results (in multiples of the limit).

Test if a concept is a member of the reference set

GET [fhir]/ValueSet/$validate-code
?system=http://snomed.info/sct&code=537131010000109 &url=http://snomed.info/sct/[moduleId]/version/[effectiveTime]?fhir_vs=refset/[refsetId]
for example
GET [fhir]/ValueSet/$validate-code
?system=http://snomed.info/sct&code=537131010000109 &url=http://snomed.info/sct/900000000000207008/version/20200131?fhir_vs=refset/635111010000100

Returns a JSON object with a "Parameters" resource, including a "result" parameter with a boolean value (true/false) indicating whether the member validated OK or not against the reference set.

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