Normal forms for description logic expressions of clinical concepts in SNOMED RT

Modern clinical terminologies organize concepts into multi-hierarchy structures that are defined by logic-based expressions, enabling compositional representation of clinical statements and supporting more complete and consistent retrieval of clinical data. The Systematized Nomenclature of Medicine, Reference Terminology (SNOMED RT) gives each concept code a semantic definition stated in description logic. The process of development, testing and distribution of these definitions has highlighted the fact that a concept definition may take many different but logically equivalent forms, and has revealed a need for a set of normal forms for authoring, distribution, and other purposes. This paper describes the difference between a choice of syntax and a choice of normal form, and defines several different normal forms, including a short canonical form, a long canonical form, and a distribution normal form.