snomed-lookup

name	snomed-lookup
description	Search SNOMED CT via the public tx.fhir.org $expand endpoint. Use when LOINC isn't the right vocabulary (problems, procedures, body sites, social context). Knows when SNOMED beats LOINC and when to use both side-by-side.

SNOMED lookup

When to use this

• The question is about a problem/diagnosis, procedure, body structure, or social-context concept (LOINC doesn't cover these well).
• An answerOption set should be SNOMED-coded (e.g. "type of headache: migraine | tension | cluster | ...").
• A boolean question has a SNOMED finding code that captures the semantic ("dizziness on standing" = SNOMED 271594007).

How to do it

tx.fhir.org is the IHTSDO public endpoint:

• https://tx.fhir.org/r4/CodeSystem/$lookup?system=http://snomed.info/sct&code=<sctid>
• https://tx.fhir.org/r4/ValueSet/$expand?url=<valueset-url>&filter=<text>

For a free-text search by display, use the $expand endpoint with url=http://snomed.info/sct?fhir_vs and &filter=<text>.

When SNOMED beats LOINC

• Diagnoses / problems: SNOMED.
• Procedures: SNOMED (or CPT in US billing context — neither in a Questionnaire usually).
• Body sites: SNOMED.
• Findings / signs / symptoms: SNOMED is broader than LOINC.

When LOINC beats SNOMED

• Lab orders / lab results: LOINC.
• Survey instruments: LOINC has explicit panel codes.
• Radiology / cardiology study types: LOINC.

When you need both

Questionnaire.code can be LOINC (for the instrument), while item.answerValueSet can point to a SNOMED-bound ValueSet for the choice options. Both is a common pattern.

Pitfalls

• SNOMED licensing: free for affiliated countries (US, UK, AU, NL, many more) but NOT universal. Surface the requirement when authoring forms intended for a non-affiliated jurisdiction.
• Pre-coordinated vs post-coordinated SNOMED expressions. v1 only uses pre-coordinated SCTIDs.
• Don't normalize concept IDs to friendly URLs; SNOMED uses the numeric SCTID (http://snomed.info/sct + <sctid>).