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The following sections outline the main features of the structure of SNOMED CT and particular aspects of interest to translators. More detailed information on the structure of SNOMED CT can be found in the SNOMED CT Guides (https://confluence.ihtsdotools.org/display/DOC/Guides).

3.1 Concept systems and ontologies

Conceptual relationships can be coded in formal structures or so-called ontologies. The construction of ontologies is based on conceptual networks derived from expert knowledge. The aim is to represent domain-specific knowledge (i.e. the conceptualization of a particular domain) in a formal, computer-readable and consensus-based way in accordance with criteria agreed upon by a community of experts. The elements of knowledge are defined according to a set of properties and linked via explicit relationships of different kinds (both hierarchical and non-hierarchical such as partitive, cause-effect, etc.). Since the concepts and relations are represented formally, the information can be processed electronically, allowing different kinds of operations such as data retrieval and standardized data exchange.

SNOMED CT is a terminological ontology that combines the terminological approach of concept system creation described in Section 2.2 with knowledge-based ontology design. It is developed by domain experts for domain experts as knowledge resource for information exchange for use in various applications. The system uses a relationship-based representation of concepts (Concept 1 <relation> Concept 2) by means of the formal ontology language Description Logic (e.g. Common Cold <causative agent> Virus).

3.2 SNOMED CT definitions

In SNOMED CT concepts are defined in three different ways.

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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

3.3 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).

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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)

3.4 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.

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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.

3.5 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.

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              363698007 |finding site|  =  66754008 |appendix structure|

3.6 SNOMED International Editorial Guide and Confluence Templates

SNOMED International has numerous resources to promote the understanding and use of SNOMED CT.  These documents are available in the Document Library, which is a collection of SNOMED CT documents, including introductory material, practical guides, technical specifications and reference material (https://confluence.ihtsdotools.org/display/DOC/SNOMED+CT+Document+Library).

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