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A concept created in an extension must conform to the logical design of SNOMED CT concepts. An overview of the logical model is provided in
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. This means that every concept created in an extension must have:
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Additional defining relationships may include
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relationships and attribute relationships. When specifying attribute relations for an extension concept it is important to ensure compliance with the SNOMED CT concept model. The concept model specifies the attributes that can be applied to particular types of concepts and it specifies the permitted values for each of these attributes. There are also additional rules on the cardinality and grouping of particular types of relationships. Please see the
SNOMED CT Editorial Guide and the
SNOMED CT Machine Readable Concept Model for details on the concept model rules.
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Logical Model Overview |
Retaining Referential Integrity
As discussed in module dependencies, all extensions have modules which depend on the modules in the International Edition. Extensions may also have modules which depend on the modules from other extensions. Despite the dependencies between modules from various editions, all concepts in an extension must be a subtype of the root concept,
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. This ensures that a concept in an extension can be queried and retrieved in a similar way to any concept in the International Edition. Put another way, queries which use subsumption should be able to identify a concept which belongs to a module in an extension because it is a subtype of a concept from the International Edition.
| Extension concepts must be a subtype of the SNOMED CT root concept |
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| The diagram illustrates how all concepts from Extension B are related to concepts in the International Edition. This is because the concept is either a direct child of a concept in the International Edition or because it is subsumed by concepts in the International Edition through subtype relationships defined by concepts in Extension A. |
The referential integrity of a concept in an extension is retained by the addition of at least one
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relationship to a parent concept that is subsumed by
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. The parent may be a concept which is part of a module within the extension, another extension, or the International Edition.
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- Other concepts in the same extension module may be stated to be a subtypes of the new concept
- The organization responsible for another module that depends on this extension module may, in the future, state that some concepts in their module are subtypes of this new concept
- When a
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is applied to the contents of the module (and the set of modules it depends upon), the defining relationships associated with a new fully defined concept may lead to additional subtype relationships being inferred. These inferred subtypes may be concepts in the extension module itself or in any module on which the extension module depends (including International Edition modules). - It is also possible (although caution is advised) that the creator of the new concept might state that the concept is a supertype of a concept in the International Edition or in another module on which the extension module depends .
- it is also possible (although caution is advised) that an extension producer might add a defining relationship to a concept in the International Edition (or in another module on which the extension module depends), and this new defining relationship may lead to a variety of new relationships (including subtype relationships) being inferred by a
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.
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A good way to assess the impact of a change is to compare the
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of subtype (
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) relationships after classifying the terminology with and without the extension. As a general rule, the transitive closure of the International Edition should not be modified by classifying it together with an extension.
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The following 3 examples illustrate the impact of adding extension concepts as leaf concepts or intermediate concepts or intermediate concepts (after classification). The impact is considered in terms of how each addition effects subsumption testing testing using an example international concept 'G'.
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Example 1 - Primitive Extension Concept as a Leaf Concept |
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| Impact on hierarchy | A primitive extension concept, which is created as a leaf concept, must always be a subtype of an international edition concept, and will not be classified as a supertype of an international concept. |
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| Impact on subsumption | The addition of this type of content does not impact subsumption testing of concepts in the international edition, because it will remain distal in the hierarchy to all International content after classification.
In this case, the transitive closure of the International Edition will stay the same, except for the addition of the new inferred | Concept |
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relationship associated with the concept from the extension. |
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Example 2 - Extension Concept as an Intermediate Concept (No impact on subsumption) |
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| Impact on hierarchy | The definition of a concept in an extension module may result in it being classified as an intermediate concept in the SNOMED CT polyhierarchy, as illustrated in the diagram above. This scenario results in the creation of two new inferred | Concept |
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relationships in the extension, i.e. the relationships G | is a | X and X | is a | E. The inferred relationship G | is a | E (green dotted line), which is present in the international edition, becomes redundant when classified together with the extension (therefore marked grey in the diagram). |
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| Impact on subsumption | This type of intermediate extension concept does not change the results of subsumption testing between international concepts. It does, however, have an impact on which relationships from the international edition are non-redundant in the extension edition. In this example, the inferred relationship G |is a| E becomes redundant when combined with the inferred relationships from the extension. Extension producers can handle this redundancy by: |
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Example 3 - Extension Concept as an Intermediate Concept (Impact on subsumption) |
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| Impact on hierarchy | The definition of a concept in an extension module may result in it being classified as an intermediate concept in the SNOMED CT polyhierarchy, as illustrated in the diagram above. This scenario results in the creation of three new inferred | Concept |
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relationships in the extension, i.e. the relationships G | is a | X and X | is a | D and X | is a | E. The inferred relationshipG | is a | E (green dotted line), which is present in the international edition, becomes redundant when combined with the inferred relationships from the extension. Furthermore, the definition of the extension concept X results in a modification of the definition of the international concept G, as G is now a subtype of the international concept D. |
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| Impact on subsumption | This type of intermediate extension concept may change the results of subsumption testing for particular international concepts, when used within the local edition. Extension producers should therefore exercise extreme caution when introducing this type of concept addition. When using the International Edition on its own, the concept G will not be treated as a subtype of the concept D. However, users of this example extension edition will see the international concept G as a subtype of the international concept D. This means that queries over international concepts stored in clinical data will lead to different results, depending on which edition is used. Therefore, intermediate concepts of this type may have serious consequences on the comparability and interoperability across SNOEMD CT Editions. |
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Process
Concepts added to an extension are represented in a a concept file. These These concepts are part of a module which includes the namespace identifier assigned to the extension producer by SNOMED International. Each concept also requires additional components and reference set members to be defined. At a minimum, the following components and derivatives should be created:
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| Concept | A new concept identifier is allocated within the extension namespace. | The attributes of the new concept are set as follows: - id is set to the new concept identifier allocated within the extension namespace
- effectiveTime is set to the date the extension will be published
- active is set to 1 to indicate that the new concept will be active at the time of publication
- moduleId is set to the conceptId of a module that is managed by the extension producer
- definitionStatusId is set to state whether the concept is primitive or fully defined
| Stated Axiom
| Authoring the concept definition involves specifying the defining properties of the concept, i.e. stating the axioms that represent necessary conditions for the meaning of the concept. For more information, see SNOMED CT Logic Profile Specification and SNOMED CT OWL Guide. | To prepare for publication, a new row representing the concept definition is added to the OWL axiom reference set. | The attributes of the reference set member representing the stated axiom are set as follows: - id is set to the new relationship identifier allocated within the extension namespace.
- effectiveTime is set to the date the extension will be published
- active is set to '1' to indicate that the new relationship will be active at the time of publication
- moduleId is set to identify a module concept from the extension
- referencedComponentId is set to the identifier generated for the new concept
- owlExpression is set to the text of the OWL expression representing the defining properties of the new concept
| | Description | Optionally, one (or more) attribute relationship is added. The principles for adding these relationships are similar to that of adding the required subtype relationship. Please refer to 5.4.4.1 Add Relationship A description of type of | Concept |
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and at least one description of type of | Concept |
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is added. For more information on creating descriptions, please refer to 5.4.3.1 Add Description in an Extension | | Stated Relationship | At least one | Concept |
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relationship to a parent concept is added. Note that concepts may have more than one| Concept |
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relationship as SNOMED CT is polyhierarchical. | | for more information about adding relationships. | | Language Refsetreference set | The new descriptions of the concept are referenced in at least one language refset to indicate language preferences. |
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