SNOMED

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SNOMED CT is an acronym that stands for Systematized Nomenclature Of MEDicine Clinical Terrms. It’s not just an acronym, it’s also one of many clinical terminology standards that aim to facilitate the coding, retrieving, analysis, aggregation, indexing, and especially the exchanging of clinical information across health care entities. As a terminology standard it functions to decrease the variability in the way health care data is recorded and encoded. SNOMED CT was created by the merger of SNOMED RT, maintained by the College of American Pathologists (CAP), USA, and the UK’s National Health Service’s Clinical Terms Version 3 (CTV3), or Read Codes. In 2007, the International Health Terminology Standards Development Organization (IHTSDO, [www.ihtsdo.org]), a non-profit organization, bought the licensing rights to SNOMED CT from the CAP.

SNOMED CT is comprised of concepts, descriptions, or standard terms for those concepts, and relationships among those concepts. The relationships formally define the concepts. The concepts in SNOMED CT exist in a hierarchical structure, which contains 19 top-level hierarchies, or axes. Each top-level hierarchy contains sub-hierarchies that further specify a concept. The 19 top-level domains: Body structure, Clinical Finding, Context-dependant category, Environments and geographical locations, Event, Linkage concept, Observable entity, Organism, Pharmaceutical / biologic product, Physical force, Physical object, Procedure, Qualifier value, Record Artifact, Social Concept, Special concept, Specimen, Staging and scales, Substance. SNOMED CT is multi-hierarchical, that is, a single concept can exist in multiple sub-hierarchies. However, sometimes there is significant confusion when a single concept exists in more than one top-level hierarchy, as can existin in the two top-level hierarchies of Substance and Product. For example, the concept Aspirin (substance) is just that substance (acetylsalicylic acid), but the concept Aspirin (product) includes all drug products that contain aspirin.

For SNOMED CT, a concept is some clinical definition or entity that is associated with a unique ConceptID. A ConceptID is a unique, numeric identifier given to a concept. A ConceptID is permanent and has no implicit meaning. A standard term for the concept is a human readable description of a concept at some level of granularity. There are three types of descriptions for a SNOMED CT concept: Preferred Term, Fully Specified Name, and Synonym. The Preferred Term is some word or phrase that is used by clinicians to name a clinical concept. The Fully Specified Name is essentially the Preferred Term, along with a “semantic tag” as a suffix to indicate the type of concept. For example, a particular concept has the Preferred Term of “apoptosis”, the ConceptID of “20663007”, and the Fully Specified Name of “Apoptosis (morphologic abnormality)”. Synonyms are additional terms that may define the concept at the same level of granularity. There are four types of relationships that a concept can have in SNOMED CT: Defining, Qualifying, Historical, and Additional. Every concept in SNOMED CT has a defining, hierarchical relationship called IS_A, to a slightly less granular parent concept (except the grand-daddy Root Concept). The IS_A relationship is basically a parent-child relationship. For example, the concept apoptosis IS_A “morphologically altered structure” IS_A “body structure”.

A new version of SNOMED CT is released every 6 months, in January and July. There are over 300k concepts, 800k descriptions, and almost a million relationships in the latest release of SNOMED CT.