Terminology in FHIR

A code means nothing without its system

A code on its own is just a string of characters. Its meaning comes from the system that defines it. That is why FHIR always treats a coded value as a pair: a system, and a code.

A Coding has four parts

A coding has four elements. The system is a URI that identifies the code system. The version is a string for the version of that code system. The code is a string that identifies a concept as defined by the code system. And the display is a description of the concept as defined by the code system.

• system · A URI naming the code system · globe · navy
• version · The version of that code system · calendar · amber
• code · Identifies the concept · tag · slate
• display · A description of the concept · text · slate

Three ways to carry a code

FHIR uses different datatypes to carry codes. The code type carries the code only, and the system is implied by the element's definition. The coding type carries a code and the system it comes from. And the codeable concept carries a concept as plain text and, or, one or more codings.

• code · The code only; system implied · check
• Coding · A code and its system · link
• CodeableConcept · Text and, or, codings · layers

System, value set, binding

FHIR separates code systems from value sets. A code system defines the codes. A value set selects which codes are allowed in a particular place. And a binding ties an element to a value set, with a strength that says how closely the value set must be followed.

• Defines · A code system · Defines the codes. · file
• Selects · A value set · Chooses the allowed codes. · check
• Ties · A binding · Links an element to a value set. · link

Binding strength, strict to loose

A binding has a strength. Required: the concept shall be from the value set. Extensible: the concept shall be from the value set if any of its codes can apply; otherwise other codings, or text, may be included. Preferred: instances are encouraged to use these codes, but are not required to. And example: the value set only illustrates the kind of concept intended.

• Required · Shall come from the value set. · lock
• Extensible · From the set if a code fits. · gear
• Preferred · Encouraged, not required. · star
• Example · Illustrates; not expected. · pencil

A Coding, as data

Here is a coded answer on a form. Where the code comes from, which version, the code itself, and how it reads. Turn it over. This is a coding as data. The system is a made up address for the code system. The version says which version of that system. The code identifies the concept. And the display is a description of it. The code system and code are made up for teaching.

Check yourself

Pause and think. What does a required binding mean for a coded element? The answer: the concept shall come from the bound value set.

• The concept shall come from the bound value set
• The value set is only an illustration
• Instances are merely encouraged to use the codes
• The element must contain free text

Keep these in mind

A coded value is a system and a code. A coding has system, version, code and display. A code system defines the codes, and a value set selects them. And binding strength is required, extensible, preferred, or example.

• A coded value is a system and a code
• A Coding has system, version, code and display
• A code system defines; a value set selects
• Binding strength: required, extensible, preferred, example