SNOMED CT can be considered as a knowledge graph (direct acyclic graph), where each concept is represented as node in the graph structure, and the position of each node corresponds to its meaning and relationship to other concepts in the graph. These positions can be thought of as coordinates. Precoordinated concepts are predefined and accurately placed in the graph, while postcoordinated expressions require coordination into the graph by a description logic classifier. This is necessary to ensure that the combined concepts are positioned correctly in the graph to represent their relationships, and to allow for effective search and retrieval of the resulting concepts. In this context, the use of coordinates refers to the position of nodes in the SNOMED graph, and precoordination and postcoordination refer to the placement of concepts in the graph at a certain moment in time.

Concepts and Expressions

A SNOMED CT concept represents a clinical idea with a unique SNOMED CT identifier that is distributed as part of a SNOMED CT release. Concepts are linked to terms by descriptions, which provide a human-readable representation of the meaning of the concept. Additionally, they are related to each other by relationships, where each relationship represents a defining property of the concept being the source of the relationship. These defining relationships provide a machine-processable representation of the concept’s meaning.

Aside from the human-readable representation, all released SNOMED CT concepts can be represented in various ways, three of which are:

• their concept identifier, which uniquely identifies the concept
• their stated view, represented through the explicit representation of their defining properties as stated by the author of the concept
• their inferred view, which represents the formal definition of concepts as logically derived by applying a description logic classifier to the stated view

Table 2.1-1 provides an example of three different views that can be used to refer to the clinical meaning 'appendicitis' .

Table 2.1-1: Example of ways of representing the meaning conveyed by the concept 'Appendicitis'.

SNOMED CT Expressions

To support a standardized and computer-processable way of representing and referring to clinical meanings in SNOMED CT, irrespective of whether this involves a single concept identifier, or a combination of concept identifiers, SNOMED CT supports expressions. 

A SNOMED CT expression is a structured combination of one or more concept identifiers that represents a single clinical idea. SNOMED International distinguish between two main types of expressions, i.e. precoordinated expressions and postcoordinated expressions.  Table 2.1-1 shows examples of both types of expressions.

Table 2.1-1: SNOMED CT expression types.

Precoordinated Expression

A precoordinated expression is the simplest form of an expression. It contains a single concept identifier, and optionally one of the terms associated with the concept to make the expression human readable. For example, a precoordinated expression, which means “fracture of tibia” can be as represented as “31978002”, optionally followed by the term ‘fracture of tibia’ placed in vertical bars known as ‘pipes’. This means that all available concepts in SNOMED CT can be represented as precoordinated expressions, by using the assigned concept identifier.

Examples of precoordinated expressions are:

• 765472003 | Excision of left kidney (procedure)|
• 108365000 | Infection of skin (disorder)|
• 387458008 | Aspirin (substance)|
• 73211009 | Diabetes mellitus (disorder)|
• 423827005 | Endoscopy (procedure)|

Postcoordinated Expression

Postcoordinated expressions contain two or more concept identifiers and are used to specify a clinical meaning by referencing existing SNOMED CT concepts.

For example, the clinical meaning "Pain in left arm" can be represented by the following expression including three SNOMED CT concepts:

Postcoordinatied expressions are useful to support the authoring of new concepts in SNOMED CT, but they may also be created to support specific implementation use cases, e.g. when no precoordinated concept is available for a specific clinical meaning, or when an information model or implementation strategy requires a certain representation of the clinical meanings.

The ability to create postcoordinated expressions greatly increases the depth of detail that SNOMED CT can represent without having to include every possible specific site for every possible disorder via a released concept.