Why Terminologies Matter

The semantic interoperability problem

Two hospitals exchange a Condition resource for a patient with diabetes. Hospital A codes it as 44054006 from SNOMED CT. Hospital B codes it as E11 from ICD-10. A third system uses a local code DM2 with no system specified.

All three mean the same clinical thing, but a system receiving these resources cannot automatically know that without additional knowledge. This is the semantic interoperability problem — getting data across systems is the easy part; ensuring both sides mean the same thing is hard.

FHIR solves this through a structured terminology framework: CodeSystems define what codes mean, ValueSets select which codes are valid in a context, and bindings connect ValueSets to specific resource elements.

CodeSystem — the source of codes

A CodeSystem defines a set of codes and their meanings. Every code used in FHIR must belong to a CodeSystem, identified by a URI. The URI is the system, the code is the value — together they are unambiguous.

Notice that Condition uses both SNOMED CT and ICD-10 in the same code.coding[] array. FHIR allows — and encourages — multiple codings for the same concept from different systems. The systems using the data can consume the coding they understand.

ValueSet — selecting codes for a context

A ValueSetdoes not define codes — it selects a subset of codes from one or more CodeSystems for use in a specific context. The distinction matters: SNOMED CT has ~350,000 concepts. A ValueSet for "condition clinical status" selects exactly 6 of them.

"clinicalStatus": {
  "coding": [{
    "system": "http://terminology.hl7.org/CodeSystem/condition-clinical",
    "code": "active",
    "display": "Active"
  }]
}

// Must use one of: active | recurrence | relapse | inactive | remission | resolved
// Defined in ValueSet: http://terminology.hl7.org/ValueSet/condition-clinical
// Binding strength: required — any other code is a validation error

Binding — connecting ValueSets to elements

A binding in a profile (or in the base spec) connects a ValueSet to a specific element. The binding strength defines how strictly the ValueSet must be followed:

SNOMED CT vs LOINC — what each is for

The most common confusion in clinical terminology is when to use SNOMED CT and when to use LOINC. They are not interchangeable.

When to create a custom CodeSystem

Sometimes no standard CodeSystem covers the concept you need — a country-specific identifier type, an institutional ward code, a local classification. In that case you define a custom CodeSystem under your domain URL and publish it as a FHIR resource.

In FHIR.sk, the FhirSkIdentifierType CodeSystem (https://fhir.sk/fhir/CodeSystem/identifier-type) defines three custom identifier types: nationalId, hospitalNumber, insuranceId. This is a synthetic example — the pattern is the same in real IGs.