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(2) Each concept also has a formal concept definition that provides a computer-processable representation of the meaning of the concept. Conceptual relationships are generated in two ways, either by human users proposing a new concept or automatically by the SNOMED CT OWL description logic classifier. The manually created representation is referred to as “stated view” (Figure 3).
Figure 3 – Diagrammatic representation of the relationships as “stated view” of the concept 195216008 |Left sided cerebral hemisphere cerebrovascular accident (disorder)| (International Edition 2021-07-31)
The automatically generated representation is referred to as “inferred view” (Figure 4).
Figure 4 – Diagrammatic representation of the relationships as “inferred view” of the concept 195216008 |Left sided cerebral hemisphere cerebrovascular accident (disorder)| (International Edition 2021-07-31)
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(3) Textual definitions that describe the meaning of a concept in natural language (see Section 3.4 and Figure 5). Only a minority of concepts have a textual definition.
Figure 5 – Illustration of the three SNOMED CT Description types in the US English language refset of concept 609328004 |Allergic disposition (finding)| (International Edition 2021-07-31) F - Fully Specified Name; S - Synonym; D - Definition
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SNOMED CT contains more than 350,000 concepts from clinical and non-clinical concept fields or domains (e.g. Body Structure, Clinical Finding, Procedure, Substance, Event, Environment or geographical location, Physical object, etc.). These concepts are arranged in 19 top-level hierarchies according to the domain to which they belong (Figure 6).
Figure 6 – The 19 SNOMED CT top level hierarchies as displayed in the SNOMED International browser (https://browser.ihtsdotools.org) taxonomy view
At the top of the SNOMED CT hierarchy is the root concept (|SNOMED CT concept|), which represents the terminology itself. All other concepts are derived from this root concept through at least one sequence of is-a relationship. This means that the root concept is a supertype of all other concepts and all other concepts are subtypes of the root concept. The direct subtypes of the root concept are called “Top Level Concepts”; each of these Top Level Concepts, together with its many subtype descendants, forms a branch of the hierarchy that contains similar types of concepts and also names the branches of the subtype hierarchy. For example, the intermediate concept 788951001 |Hemorrhage of digestive system (disorder)|, which has 17 children, is itself a type of |Disease| which is a type of |Clinical finding|. The further down the hierarchy, the more granular, or in other words more precise, a concept is. This principle is referred to as granularity (Figure 7).
Figure 7– Granularity (SNOMED International browser (https://browser.ihtsdotools.org) summary view, International Edition 2021-07-31)
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As many clinical concepts are multidimensional by nature, concepts can have more than one parent. This creates a polyhierarchical structure, which increases the amount of information provided. As the formal representation language of SNOMED CT uses both hierarchical and attributive relationships, the multidimensional character of concepts can be expressed accordingly. Figure 8 shows the polyhierarchical structure of SNOMED CT concepts.
Figure 8 – Polyhierarchical structure of concepts (as shown in SNOMED International Starter Guide)
Figure 9 – Full polyhierarchical view of concept 788951001 |Hemorrhage of digestive system (disorder)| (International Edition 2021-07-31) using Ontoserver Shrimp browser (https://ontoserver.csiro.au/shrimp)
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Each concept is represented at least by two types of Descriptions: one Fully Specified Name (FSN) and Synonyms (SYN), one of which is marked as the Preferred Term (PT) (Figure 10). The presence of a Description of type definition is optional.
Figure 10 – SNOMED CT descriptions
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