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A Relationshiprepresents an association between two Concepts.

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

A Relationshipcontains Identifiersof two logically associated Conceptsand the Identifierof another Conceptthat indicates the Relationship Typeby which they are associated.

Table 3.1.3-1: Example: Defining arthritis as a type of joint disorder

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Relationships and concept definitions

Each conceptin SNOMED CTis logically defined through its relationshipsto other concepts.

Every active SNOMED CT concept(except the SNOMED CT Concept Root concept) has at least one 116680003 |is a| relationshipto a supertype concept. 116680003 |is a| relationshipsand defining attribute relationshipsare known as the defining characteristicsof SNOMED CT concepts. They are considered defining because they are used to logically represent a conceptby establishing its relationshipswith other concepts. This is accomplished by establishing 116680003 |Is a| relationshipswith 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 relationshipis assigned only when that relationshipis 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| relationshipsare also known as "Supertype - Subtype relationships" or "Parent - Child relationships". 116680003 |is a| relationshipsare the basis of SNOMED CT's hierarchies, as illustrated below.

Figure 3.1.3-1: Example IS A hierarchy

A conceptcan have more than one 116680003 |is a| relationshipto other concepts. In that case, the conceptwill have parent conceptsin more than one sub-hierarchyof a top-level hierarchy. Subtype relationshipscan be multi-hierarchical.

Figure 3.1.3-2: Example IS A Relationships

Attribute Relationships

An attribute relationshipis an association between two conceptsthat specifies a defining characteristicof one of the concepts(the source of the relationship). Each attribute relationshiphas a name (the type of relationship) and a value (the destination of the relationship).

The combination of the attribute relationshipsand 116680003 |is a| relationshipsassociated with a conceptrepresent the logical definition of that concept. Therefore, the logical conceptdefinition includes one or more supertypes (represented by 116680003 |is a| relationships), and a set of defining attributesthat differentiate it from the other conceptdefinitions.


Since pneumonia is a disorder of the lung, the logical definition of the concept 233604007 |Pneumonia (disorder)|in SNOMED CTincludes 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 Attributewith 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)|

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

Figure 3.1.3-6: Illustration of Defining Relationships