This section summarizes the ways in which SNOMED CT concept definitions are represented. It outlines the rationale for changes to the way in which the stated view of concept definitions is represented. The changes are occurring during a transitional period that started in July 2018 and is due to be completed in 2019.

As illustrated in Figure 2.3.4.1-1, each defining relationship is represented by a row in the relationship file. The concept being defined is referenced by the sourceId, the concept that represents the type of relationship (attribute) is referenced by the typeId and the destinationId refers to the concept that represents the value of that attribute.

The relationship file also has a relationshipGroup which allows two or more defining relationships to be grouped together.

The definitionStatusId of the source concept, indicates whether the combination of defining relationships provide provides sufficient definition of that concept.

Example of stated view of |Excision of stomach structure| represented by stated relationships

Section , illustrated the following three points, which are not supported by the current use of relationships to represent concept definitions:

1. A concept may have more than one sufficient definition.

   • Use of relationships only supports representation of a single sufficient definition for each concept. If a concept is marked as sufficiently defined, all it relationships are considered to be part of its sufficient definition.

2. A concept may have a sufficient definition that includes some assertions that are not necessary conditions

Section , identifies other useful features that are supported by description logic tools but cannot be represented using only SNOMED CT.

Relationships can be distributed in an easy to understand relational file structure. The relationship file has been an established part of the standard set of SNOMED CT release files since the first release in 2002, with a revision in 2011-2012 to use RF2 to enhance versioning capabilities. Relationships can be retrieved, displayed and processed using widely understood techniques such as SQL making it easy to join the relationships to the concepts to which they relate.

The stated view of concept definitions needed to be enhanced to allow more flexible and expressive use of description logic. The structure of the relationship file was not suitable for this and a decision was made to adopt the OWL Functional Syntax so that new DL features could be added over time. As a result, the stated relationship file was deprecated.

Information about the new representation for the stated view is included in section .

The current relationship file continues to be released containing the inferred view. Due to limitations of the relationship file format, the inferred definitions do not contain the more sophisticated DL features. The relationship file:

• only contains necessary conditions

• does not distinguish between multiple sufficient definitions, regardless of whether each necessary condition is part of any of the sufficient definitions.

Nevertheless, the end result is still more complete and precise than the previous content of this file. The reason for this is that the inferred relationships in the file is generated by processing the enhanced stated view. Details of the way the inferred relationships are generated from the stated view are documented in .

This section outlines the rationale for distributing an OWL representation of the stated view of concept definitions and provides an overview of the way OWL axioms are represented in SNOMED CT release files. More detailed information is published separately in the SNOMED CT OWL Guide and the SNOMED CT Logic Profile Specification.

Rationale for Using OWL

The mismatch between the requirements for representing enhanced concept definitions and the capabilities of the current stated relationship file might in theory be addressed by addition of columns to the file or adding additional information in reference sets. However, in practice this would create a more complex solution able to support a specific set of enhanced features.

Adopting the well-established OWL standards formats offers a more flexible solution that can represent the full range of description logic features. This approach enables SNOMED Internation to specify a particular logic profile to be applied to current releases of SNOMED CT, with the option to extend that profile in the future. Future revisions of the logic profile would not require a change in the distribution file structure provided these were supported by an OWL syntax.

OWL Axioms

OWL axioms can be represented using several different syntaxes. SNOMED International has chose the OWL Functional Syntax as its standard representation.

The OWL Functional Syntax is a formal representation of the web ontology language (OWL) as a simple text base syntax that is used as a bridge between the structural specification and various concrete syntaxes.

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OWL axioms are distributed in a reference set that follows the specification. The axiom itself is contained in a string field and the concept whose definition it contributes to is referenced by the the referencedComponentId. Although a single row in the reference set can provide a sufficient definition, the definition of a single concept can also include several axioms each represented by a row in the reference set.

Figure 2.3.4.2-1 shows the diagrammatic representation of the stated view of the definition of . Table 2.3.4.2-1 shows the same definition as represented by three rows in the stated relationship file together with the definitionStatusId in the concept file.

Stated relationships and definition status for the sufficiently defined concept

Example of OWL axiom refset representation of the definition of |Excision of appendix|.

The above table shows a row in the OWL axiom reference set file representing the same definition. As shown by this example, a single sufficient definition is represented by a single row in the reference set. However, some concept definitions may require multiple rows in the reference set. Situations in which multiple row are required include:

• Concepts with multiple sufficient definitions, each of which requires a separate row in the reference set.

• Concepts with additional necessary conditions that are not part of a sufficient definition, each of which requires a separate row in the reference set.