Postcoordination

Postcoordination is a method used in clinical terminologies to express detailed clinical concepts by combining two or more individually defined concepts after the original coding event. It contrasts with precoordination, where a single code already encodes the full concept. Postcoordination is commonly applied within SNOMED CT, though the general idea appears in other ontologies as well. It enables clinicians and information systems to represent nuanced conditions, procedures, findings, and modifiers that are not captured by a single code.

A postcoordinated expression is built by selecting a base concept and linking it to additional concepts through defined relationships or modifiers. This may specify attributes such as site (anatomical location), laterality, severity, causation, temporality, or morphology. For example, a fracture of the left radius with displacement can be represented by the base concept for the fracture and relationships indicating left side and displaced; or a diagnosis like pneumonia with a specific causative organism can be postcoordinated as disease plus causative agent.

Benefits include finer clinical detail, improved data granularity, and enhanced decision support, reporting, and interoperability when systems share expression rules. Challenges include increased complexity, the need for governance and training, potential inconsistency in expression patterns, and the requirement for tooling to create, store, and interpret postcoordinated expressions. Standards like SNOMED CT provide guidelines and tooling, including expression constraint languages, to facilitate consistent postcoordination. In practice, postcoordination supports richer clinical documentation and more precise data capture in electronic health records.