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Relationships and concept definitions
Every activeSNOMED CT concept(except the SNOMED CT ConceptRoot concept) has at least one 116680003 |is a|relationship to a supertype concept. 116680003 |is a|relationships and defining attribute relationships are known as the defining characteristics of SNOMED CT concepts. They are considered defining because they are used to logically represent a concept by establishing its relationships with other concepts. This is accomplished by establishing 116680003 |Is a|relationships with one or more defining concepts(called supertypes) and modeling the difference with those supertypes through defining attributes.
Example: 263245004 |Fracture of tarsal bone (disorder)|is defined as:
Example: Group A Streptococcus causes most cases of Streptococcal pharyngitis. However, a small percentage of these cases are caused by other species of Streptococcus. Therefore, it would be incorrect to define 43878008 |Streptococcal sore throat (disorder)| as having 246075003 |causative agent| 80166006 |Streptococcus pyogenes (organism)|. Instead it is correctly defined as having the more general 246075003 |causative agent| 58800005 |Genus Streptococcus (organism)|.
116680003 |is a|relationships are also known as "Supertype - Subtyperelationships" or "Parent - child relationships". 116680003 |is a|relationships are the basis of SNOMED CT hierarchies, as illustrated below.
Figure 3.1.3-1: Example IS A hierarchyA concept can have more than one 116680003 |is a|relationship to other concepts. In that case, the concept will have parent concepts in more than one sub-hierarchy of a top-level hierarchy. Subtyperelationships can be multi-hierarchical.
Figure 3.1.3-2: Example IS A Relationships
An attributerelationship is an association between two concepts that specifies a defining characteristic of one of the concepts(the source of the relationship). Each attributerelationship has a name (the type of relationship) and a value (the destination of the relationship).
The combination of the attributerelationships and 116680003 |is a|relationships associated with a concept represent the logical definition of that concept. Therefore, the logical concept definition includes one or more supertypes (represented by 116680003 |is a|relationships), and a set of defining attributes that differentiate it from the other concept definitions.
Since pneumonia is a disorder of the lung, the logical definition of the concept 233604007 |Pneumonia (disorder)| in SNOMED CT includes the following relationship. The Attribute 363698007 |Finding site| is assigned the value 39607008 |Lung structure (body structure)|.
The full definitions of the concepts 233604007 |Pneumonia (disorder)|, 312342009 |Infective pneumonia (disorder)| and 53084003 |Bacterial pneumonia (disorder)| are shown below. The first lines in each example refer to supertypes of the defined concept. Each of the subsequent lines refines the definition by specifying an attribute with a value.
Figure 3.1.3-3: Definition of |Pneumonia (disorder)|
Figure 3.1.3-4: Definition of |Infective pneumonia (disorder)|
Figure 3.1.3-5: Definition of |Bacterial pneumonia (disorder)|Figure 3.1.3-6 illustrates some of these Relationships graphically. 116680003 |is a|Relationships relate a concept to more general concepts of the same type. In contrast, AttributeRelationships (such as 363698007 |Finding site| and 246075003 |Causative agent|) relate a concept to relevant values in other branches of the subtype hierarchy.
Figure 3.1.3-6: Illustration of Defining Relationships