← ReferenceTerminology
What is a CodeSystem?
A CodeSystem defines a set of codes and their meanings. Every code used in FHIR belongs to a CodeSystem, identified by a URI. CodeSystems are the source of codes; ValueSets select subsets of them.
Major CodeSystems in healthcare
| CodeSystem | URI | Used for |
|---|---|---|
| SNOMED CT | http://snomed.info/sct | Diagnoses, findings, procedures, anatomy |
| LOINC | http://loinc.org | Lab tests, observations, panels, vitals |
| ICD-10 | http://hl7.org/fhir/sid/icd-10 | Diagnoses (billing, epidemiology) |
| ATC | http://www.whocc.no/atc | Medications by anatomical/therapeutic class |
| RxNorm | http://www.nlm.nih.gov/research/umls/rxnorm | US drug codes (clinical drugs) |
| UCUM | http://unitsofmeasure.org | Units of measure (mg, mmol/L, kg) |
Custom CodeSystems
When no standard CodeSystem covers a concept (e.g. an institution-specific ward type or a country-specific reason code), you define a custom CodeSystem with a URL under your domain — e.g. https://fhir.sk/fhir/CodeSystem/ward-type. Custom CodeSystems must be published as FHIR resources so validators can look them up.