Value set

What it is

A value set selects codes from one or more code systems, such as SNOMED CT, LOINC or ICD, for a specific use. The code system defines the concepts; the value set says which ones can be used in a given context. FHIR takes these concepts from the HL7 v3 Core Principles, so the idea predates FHIR and does not depend on it.

Definition and expansion

A value set has two aspects:

• Definition (compose, the "intension"): the rules that state which codes it includes.
• Expansion (the "extension"): the list of codes it actually contains at a given time and with certain terminology versions.

Depending on how it is defined, it can be:

In FHIR, the $expand operation asks a server to compute the expansion, and $validate-code checks whether a code belongs to the set.

Binding to a data element

A value set is bound to an element of a data model with a strength. FHIR defines four:

• required: the code must come from the set.
• extensible: it must come from the set if any code applies; if none covers the concept, others are allowed.
• preferred: using it is encouraged but not mandatory.
• example: it only illustrates the kind of concepts expected.

Common misconceptions

• A value set is not a terminology: it does not create concepts, it selects them.
• The expansion of an intensional set changes when the terminology version changes. For reproducible results, pin the versions.
• Binding strength matters: a validator rejects a code outside a "required" set, but not outside a "preferred" one.
• Whoever publishes a value set must maintain it. An abandoned set keeps circulating with obsolete codes.