Post-coordination & expressions
When no single code says it, compose one: pre-coordinated codes vs. expressions built from parts, in compositional grammar.
The answer in one paragraph
Most of the time you record a pre-coordinated concept — a single existing code like
Fracture of femur. When the precise meaning doesn't exist as one code (left side?
caused by what? severity?), SNOMED CT lets you post-coordinate: compose an
expression from a base concept plus attribute–value refinements, written in
Compositional Grammar (SCG). The expression means something the terminology can
still classify and query — you're not inventing a local code, you're speaking the same
language in longer sentences.
The grammar in one example
71620000 |Fracture of femur| :
272741003 |Laterality| = 7771000 |Left|
Read it as: the concept Fracture of femur, refined so that its laterality is left.
The colon introduces refinements; each refinement is attribute = value; both sides are
themselves concepts. Multiple refinements separate with commas; grouped attributes wrap
in { } when several belong together (e.g., one finding site with its own morphology).
Pre vs. post: how to choose
| Pre-coordinated | Post-coordinated | |
|---|---|---|
| What | One existing concept ID | Expression composed at recording time |
| Pros | Simple to store, query, exchange | Says exactly what happened |
| Cons | May not exist for your exact meaning | Systems must store/classify expressions |
| Rule of thumb | Always prefer it when it exists | Use when precision genuinely matters |
In the real world, adoption of post-coordination is uneven — many EHRs store only
pre-coordinated codes, and interfaces (including FHIR's CodeableConcept) carry
expressions less commonly. Knowing the grammar still pays: it's how the terminology
itself defines concepts, and it's the foundation ECL queries build on.
The attributes you're allowed to use
You can't attach any attribute to any concept: the concept model (Machine Readable
Concept Model, MRCM) defines which attributes apply to which domains — Laterality
applies to body structures; Causative agent to clinical findings; Method to
procedures. Authoring tools (and Shabda Studio, later) validate against these rules so
expressions stay classifiable.
Why a practitioner should care
- Reading definitions: open any concept and its defining relationships are exactly this grammar — you can now read them.
- Edge-case capture: registries and research often need the precision single codes lack.
- Mapping work: understanding composition explains why some local terms map to expressions rather than single SNOMED codes.
Where to go next
ECL — querying SNOMED turns the same grammar into a one-line query language over the whole graph.