Most extensions include additional concepts, descriptions and/or relationships. It is therefore important for extension producers to understand the logical design of these components, and any extension-specific rule related to these components.

The following subsections present logical design rules and considerations for components in an extension:

• Common Attributes

• Concepts

• Descriptions

• Relationships

For information about the process of authoring extension components, please refer to . For a full list of RF2 component tables and their attributes, please refer to the SNOMED CT Release File Specifications.

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Common Attributes

All components in an extension are represented using the same logical design as the International Edition. Concepts, descriptions and relationships all share four common attributes:

• id

• effectiveTime

• active

• moduleId

The following subsections explain how these attributes are used in an extension.

Id

The first attribute of every extension component is 'id '. The id uniquely identifies the component across all extensions globally. It is therefore important that extension identifiers are structured according to agreed rules. The image below shows the four parts of an extension SNOMED CT identifier (SCTID) - item identifier, namespace identifier, partition identifier and check digit.

The table below explains the rules related to each of the four parts of a SNOMED CT identifier in an extension.

Table: Rules for each part of an extension SCTID

effectiveTime

The second attribute of every extension component is 'effectiveTime'. This attribute specifies the date on which the specific version of the component was released, using the ISO 8601 YYYYMMDD format. When a component is added or modified in an extension, the effectiveTime of the component version will match the version date of the release.

active

The third attribute of every extension component is 'active '. This attribute specifies whether or not the specific version of the component is active at the given effectiveTime. The data type of this attribute is Boolean, with a value of "1" indicating that the component is active at the given effectiveTime and a value of "0" indicating that the component is inactive at the given effectiveTime. New components in an extension will use an active value of "1".

moduleId

The fourth attribute in every extension component is 'moduleId '. This attribute specifies the module in which the component is being maintained at the given effective time. The value of this attribute is the SCTID of a module concept that is created and maintained by the extension producer. Please refer to Module Definition for more information on this topic.

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Overview

Concepts can be added to an extension, either to address content gaps or to expand the scope of SNOMED CT to include concepts that are used locally. Extension concepts are distributed in a , which follows the standard RF2 release file format. For more details on the principles and processes for authoring concepts in an extension, please refer to Authoring Concepts.

Attributes

In addition to the four common attributes, extension concepts also have a definitionStatusId.

definitionStatusId

The definitionStatusId attribute specifies whether the concept is primitive or fully defined. If defining relationships are added, which sufficiently define the concept, then the definitionStatusId should be set to | Defined| . Otherwise, the extension concept is | Primative| .

Example

In the image below, a row from the concept table of the 20170901 US edition is shown, which represents the concept 5281000124103 | Persistent asthma| .

Note that the attribute values in this row have been set according to the rules previously described.

Please note the following:

• The concept id uses a namespace identifier allocated to the National Library of Medicine (NLM). This indicates that this concept was originally created by the NLM.

• The concept id uses a partition identifier of “10”. This indicates that the concept was created in an extension.

• The moduleId indicates that this concept is included in the | US National Library of Medicine maintained module| , which is a module in the US Edition.

• The definitionStatusId states that this concept is | Primitive| .

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Descriptions

Overview

A description may be created in an extension as part of the process of adding a new concept, or adding a translation or alternative synonyms to an existing concept. Extension descriptions are distributed in a which follows the standard RF2 release file format. For more details on the principles and processes for authoring descriptions in an extension, please refer to Authoring Descriptions.

Attributes

In addition to the four common attributes, descriptions also have a conceptId, languageCode, typeId, term and caseSignificanceId.

conceptId

The conceptId attribute links the description to the concept that it describes. The conceptId may refer to a concept in the same extension module or to a concept in a module on which the extension module depends.

languageCode

The languageCode is a two-character string, which specifies the language of the description using the ISO 639-1 standard (e.g. 'EN', 'ES'). Please note that the languageCode specifies only the language of the description (e.g. English or Spanish) and does not indicate the dialect (e.g. US English or GB English). Dialect preferences are represented by language reference sets. Extensions may use a languageCode not used by the International Edition.

typeId

The typeId is a SCTID that specifies the type of the description. The value of this attribute must be a descendant of | Description type| . Possible values include | Fully specified name| , | Synonym| , and | Definition| . Note that | Definition| is used for text definitions, which are not included in the main description distribution files. Instead, descriptions of type | Definition| are distributed in a separate text definitions file.

term

The term attribute is a text string that describes the given concept. The term is represented using UTF-8, which allows non-English character sets to be used, such as the accented characters used in Nordic languages, Hindi characters and Chinese characters. The maximum length of a fully specified name or synonym is defined in the | Description format reference set| as 255 characters. Longer terms can be used for text definitions, which may be up to 4096 characters in length.

caseSignificanceId

The caseSignificanceId attribute is a SCTID which specifies how the case of the characters in the term may be changed when using the term. The values of this attribute always refer to descendants of the concept | Case significance| , and at the time of this publication, the three possible values are | Entire term case insensitive| , | Entire term case sensitive| ,or | Only initial character case insensitive| .

Examples

In the image below a row is presented from the descriptions table of the 20170901 US edition. Note that this description is associated with the extension concept from the previous example.

Please note the following:

• The description id uses a namespace identifier allocated to the National Library of Medicine (NLM). This indicates that this description was created by the NLM.

• The description id uses a partition identifier of “11”. This indicates that the description was created in an extension.

• The moduleId indicates that this description is included in the | US National Library of Medicine maintained module| , which is the same module that the associated concept (from the previous example) is included in.

• The description id and the conceptId both use a namespace identifier owned by the NLM. This indicates that this description is associated with a concept created by the same extension producer.

• A languageCode of "en" indicates that this description uses the English language.

• The typeId indicates that this description is a .

• The term indicates that the string "Persistent asthma" is used to describe the concept .

• The caseSignificanceId indicates the term is . This means that the term can be displayed using any alternative casing, such as "PERSISTENT ASTHMA" and "persistent asthma".

In the image below a row is presented from the descriptions table of the 20170430 Canadian edition. Note that this description is associated with a concept from the International Edition.

Please note the following:

• The description id uses a namespace identifier allocated to Canada Health Infoway (CHI). This indicates that this description was created by the CHI.

• The description id uses a partition identifier of “11”. This indicates that the description was created in an extension.

• The moduleId indicates that this description is included in the | Canada Health Infoway French module| .

• The conceptId refers to the concept 1475003 | Herpes labialis| from the International Edition. This indicates that this term may have been created as a language specific term for an international concept.

• A languageCode of "fr" indicates that this description uses the French language.

• The typeId indicates that this description is a .

• The term indicates that the string "herpès labial (trouble)" is used to describe the concept . Please note the semantic tag, "trouble" is a French word for "disorder".

• The caseSignificanceId indicates the term is . This means that the term can be displayed as either "Herpès labial (trouble)" or "herpès labial (trouble)".

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Relationships

Overview

Relationships are added in an extension to link a new concept to one of its supertypes, or to specify defining attribute relationships. Note that at least one hierarchical ( | Is a| ) relationship is required for each new extension concept. Extension relationships are distributed in a , which follows the standard RF2 release file format. For more details on the principles and processes for authoring relationships in an extension, please refer to Authoring Relationships.

Attributes

In addition to the four common attributes, relationships also have a sourceId, destinationId, relationshipGroup, typeId, characteristicTypeId and modifierId.

sourceId

The sourceId identifies the concept being defined by the relationship. In the case of a hierarchical ( | Is a| ) relationship, the sourceId refers to the subtype/child concept.

destinationId

The destinationId represents the value of the defining relationship. In the case of a hierarchical ( | Is a| ) relationship, the destinationId refers to the supertype/parent concept. The destinationId may refer to a concept in the same extension module as the source concept, or any module on which this extension module depends.

relationshipGroup

The relationshipGroup is an integer that indicates whether or not the relationship is grouped together with other relationships associated with the same source concept. Any relationships with the same sourceId and relationshipGroup integer (other than zero) are considered to belong to the same relationship group. A relationshipGroup of zero (i.e. "0") indicates that the relationship was not grouped with other relationships by the terminology author. For more information on relationship groups, please refer to Relationship groups in SNOMED CT.

typeId

The typeId is a SCTID that specifies the attribute type of the relationship. In the case of a hierarchical relationship, the value of this attribute is | Is a| . Attribute relationships use a typeId value that refers to a subtype of | Concept model attribute| . Common examples of attribute types include | Finding site| and | Method| .

In most cases, the typeId will refer to an international concept. However, additional attribute concepts may be added in an extension if there is a legitimate clinical need, and clear rules and guidance is provided. For more information, please refer to Add Relationship in an Extension.

characteristicTypeId

The characteristicTypeId is a SCTID, which specifies whether this relationship was stated by an author or was inferred by a description logic classifier. For more information, please refer to Classifying an Edition.

modifierId

The modifierId attribute is a SCTID which specifies the type of description logic that applies to the given relationship - existential restriction (i.e. 'there exists some') or universal restriction (i.e. 'there exists only'). All relationships in the International Edition use the value | Existential restriction modifier| . This value is also recommended for extension relationships, as the value | Universal restriction modifier| can significantly affect the speed of classification.

Example

In the image below two rows from the relationship table of the 20170901 US edition are shown. Note that these relationships are used to define the extension concept from the earlier example.

Please note the following:

• The relationship ids use a namespace identifier allocated to the National Library of Medicine (NLM). This indicates that these relationships were originally created by the NLM.

• The relationship ids use a partition identifier of “12”. This indicates that the relationships were created in an extension.

• The moduleId indicates that these relationships are included in the | US National Library of Medicine maintained module| . This is the same module as the source concept belongs to.

• The sourceId indicates that the relationship defines the concept | Persistent asthma| .

• The typeId indicates that the first relationship is an | Is a| relationship, while the second relationship specifies the | Finding site| of the source concept.

• The destinationId indicates that | Asthma| is a supertype of | Is a| , and that the | Finding site| of | Is a| is | Airway structure| .

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