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A Relationship represents an association between two Concepts.

Each Relationship is identified by a unique Relationship Id and is distributed as a row in the Relationship file.

A Relationship contains Identifiers of two logically associated Concepts and the Identifier of another Concept that indicates the Relationship Type by which they are associated.

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Relationships and concept definitions

Each concept in SNOMED CT is logically defined through its relationships to other concepts.

Every active SNOMED CT concept(except the SNOMED CT Concept Root 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:

Note: A relationship is assigned only when that relationship is always known to be true.

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)|.

IS A Relationships

116680003 |is a| relationships are also known as "Supertype - Subtype relationships" or "Parent - Child relationships". 116680003 |is a| relationships are the basis of SNOMED CT's hierarchies, as illustrated below.

A 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. Subtype relationships can be multi-hierarchical.

Attribute Relationships

An attribute relationship is an association between two concepts that specifies a defining characteristic of one of the concepts(the source of the relationship). Each attribute relationship has a name (the type of relationship) and a value (the destination of the relationship).

The combination of the attribute relationships 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)|,|Infective pneumonia (disorder)| and |Bacterial pneumonia (disorder)| are shown below. Each line represents a defining Attribute with a value.

illustrates some of these Relationships graphically. 116680003 |is a| Relationships relate a concept to more general concepts of the same type. In contrast, Attribute Relationships(such as 363698007 |Finding site|and 246075003 |Causative agent|) relate a concept to relevant values in other branches of the subtype hierarchy.

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