Extensions can be used to add content to SNOMED CT. These additions can address perceived content gaps in SNOMED CT because content is:
In some cases, a content gap will be addressed by adding content directly to the International Edition, but in other cases the content will be rejected for inclusion in the International Edition. When content is outside the scope of the International edition, yet required to support national or local needs, this content may be added to an extension. For example
As illustrated in Figure 3.2-1 below, content can be added to SNOMED CT by inclusion in the International Edition or in relevant extensions. National extensions will add the content relevant for multiple Affiliates (or stakeholders) within a country or member territory, whereas highly specific content, which is only relevant for a specific vendor product or hospital may be added in a local extension.
Figure 3.2-1: Extensions can be used to add national or local content to SNOMED CTIn some circumstances, extensions can be used to extend the expressivity of SNOMED CT, for example by expanding the concept model or by specifying additional terminology characteristics and features. SNOMED CT enables the following enhancements to expressivity to be made by adding terminology metadata.
Once the required metadata has been added, the enhancement can be used within the extension and can be applied to SNOMED CT components in the extension, the International Edition, or any other modules on which the extension depends.
Extending the expressivity of SNOMED CT does not necessarily require the creation of new clinical content in SNOMED CT. However, it does require the addition of concepts within the | SNOMED CT Model Component| hierarchy of SNOMED CT and in some cases the addition of reference set members to one of the metadata reference sets.
In section 5 Key Steps we explain the full set of steps for producing any extension. However, not all of these steps are required for all purposes. If producing an extension for the purpose adding content to SNOMED CT, the following key steps are of particular relevance: