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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)
For translators it is of outmost importance to look also at the formal definition of the concept as SNOMED CT translations should be concept-based (see Section 4). The formal definition can help to understand the meaning and to increase translation consistency.
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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
The hierarchical and multi-axial structure of SNOMED CT
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).
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7– Granularity (SNOMED International browser (https://browser.ihtsdotools.org) summary view, International Edition 2021-07-31)
The top rectangle in Figure 7 shows the hierarchy view up to the root concept for each parent |Bleeding (finding)| and |Disorder of digestive system (disorder)| of the concept 788951001 |Hemorrhage of digestive system (disorder)|. The lower rectangle shows some of the 17 child concepts, while the middle rectangle shows the concept Descriptions. The rectangle to the right shows the concept’s relationships.
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)
Associated morphology |
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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)
Description types
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
The FSN is “a description that represents the meaning of a concept in a way that is unambiguous and independent of the context in which it is used” (SNOMED International 2020). Therefore, an FSN can also be considered as a kind of definition or explanation as it describes the content of the concept as explicit as possible (see Section 3.2). If a synonym, the logical definition or the logical definition or the text definition conflicts with the FSN, the FSN is the gold standard or so-called “Source of Truth”. The FSN should be able to stand alone as complete, unambiguous & comprehensible.
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Description type | Description |
Fully Specified Name | Photodermatitis (disorder) |
Synonym (Preferred) | Photodermatitis |
Synonym (Acceptable) | Photosensitivity dermatitis |
Text definition | An abnormal inflammatory skin condition resulting from exposure to ultraviolet light, most commonly sunlight. May result from phototoxic or photoallergic reactions or both. |
Table 1 – SNOMED CT description types for the concept 22649008 |photodermatitis (disease)| in the Great Britain English language reference set
For translation purposes the three types of Definitions can be useful (see Section 4). Professional translators without medical expertise will have greater difficulty understanding all SNOMED CT by their formal relationships. Therefore, natural language definitions and textual sources that provide information on the contextual use of a concept are important additional tools.
SNOMED CT logical definitions, attributes and relationships
In the SNOMED CT ontology, each concept is logically defined through its relationships to other concepts. For the definition of a concept, attribute-value pairs are used in addition to the hierarchical “is a” relationships. An attribute represents a characteristic of the meaning of a concept to which a value is assigned.
The attributes that can be applied depend on the concept model. The concept model defines which attribute value-pairs can be applied to which (sub)hierarchy of concepts and how those attribute-value pairs need to be grouped together to ensure correct unambiguous understanding of the concept definition (see the Editorial Guide for a detailed description of concept models). For example, a procedure may have a method, and a disorder may have an etiology, but a procedure cannot have an etiology, and disorder cannot have a method. Defining characteristics represent the values of a range of relevant attributes. Depending on the nature of the concept, there are more than 100 approved attributes to be used in the concept model. The permitted range of values for an attribute depends on the rules specified in the concept model.
All SNOMED CT concepts are linked at least by one is-a relationship to the immediate superordinate(s) concept(s) and by zero to many attributive relationships. Both types of relationships together form the (formal) definition of a concept. Relationships are not limited to concepts and can also refer to numerical values in products.
There are two types of defined concepts: fully defined and primitive concepts.
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