Semantic Forms

Similar to the stated and inferred view of precoordinated concepts, as described in 2.1 Precoordination and Postcoordination, the meaning represented by postcoordinated expressions can also be represented in different ways. When enabling postcoordinated expressions within a system, it is important to be support these different views and understand why they are needed.

Close to User Form

The Close to User Form (CTU) Expression is the faithful representation of the clinical meaning as it was entered by the user. It is the{ primary stored and communicated view of the clinical information that’s encoded using SNOMED CT. The CTU Expression includes any refinement that was applied by the system, based on the selections made in a data entry form, or those made explicitly. 

And, importantly, it does not include any additional relationships that are added, based on classifier rules to make the expression complete or to normalize it. It only includes the parts of the clinical meaning that were specifically intended by the user.

Classifiable Form

The Classifiable Form (CF) expression is a syntactically valid and concept model compliant representation of the CTU expression. It serves as the input to the classifier, enabling expressions to be classified together with other SNOMED CT content.

And in the classifiable form, any ungrouped attributes and role groups have been validated and appropriately applied to the definition of the focus concept.

Necessary Normal Form

The Necessary Normal Form (NNF) expression is the inferred view of the expression, and it includes all the relationships that are necessarily true, with redundancy removed. This is the output of the process that took the classifiable form expression and classified it with a given SNOMED edition. The NNF represents the necessary relationships used for querying, so this becomes part of your substrate when you’re running an Expression Constraint query.

The NNF includes refinements that represent inferred relationships, without any redundant refinements or redundant relationship groups.

Objective of Different Semantic Forms

Within the released SNOMED CT content, the stated definitions of SNOMED CT concepts are represented as Description Logic OWL axioms, and these are the clinical definition that SNOMED authors state when they define the meaning of a concept. For postcoordinated expressions, the CTU expression can be compared to the stated concept definition, as it represents the expression as it was created.

When querying SNOMED CT concepts, the inferred definitions are used. The inferred definition of a concept is derived (or inferred) by classifying the stated definitions using a Description Logic reasoner. The output of this classification process is represented in the release using the relationship table and includes the necessary normal form for each concept. The necessary normal form represents the set of ‘necessary’ inferred relationships. For more information on concept definitions, please refer to Appendix D of the release file specification: Concept Definition Illustrations. In the same way as for precoordinated concepts, the inferred view, or the NNF expressions, needs to be generated to support querying over postcoordinated expressions, see Figure 2.4-1.

To enable the classification of the CTU expression, a transformation is required to provide a form that can serve as input to a Description Logic reasoner. Such reasoner requires a form which is syntactically valid and complies to the concept model rules. This is why all support for postcoordination also requires the ability to generate the CF of each expression.

Figure 2.4-1: Expression forms compared to the representations of precoordinated content.

Canonical Form

Although the Compositional Grammar syntax supports a standard way of representing SNOMED CT expressions, expressions that contain exactly the same concept identifiers and refinements, may still differ from one another in the following ways:

• Inclusion of whitespace between elements
• Inclusion of specific terms associated with identified concepts
• The order in which focus concepts, refinements, attributes, and attribute groups appear

The canonical form of an expression is a serialized representation produced by applying a set of rules that ensure a single unique representation for any expression.

Examples

Table 2.4-1: Examples of expressions and their canonical form.