This section provides an overview of:
SNOMED CT is designed to allow the International Edition to be enhanced by adding Extensions that address national or local requirements. Additional content required to support national, local or organizational needs that may not have international relevance or may not meet the editorial guideline for inclusion in the International Edition.
SNOMED CT design also includes the Reference Set mechanism which provides a standard way to customize and enhance content for use in a particular country,language, specialty, application or context. Reference Sets developed nationally or locally can modify search and display of content from the International Edition as well as enhancing Extension content.
Many clinical concepts are relevant in all countries, organizations and specialties but some concepts are relevant only to a particular environment. SNOMED CT is designed to allow the International Edition to be enhanced by adding Extensions to meet national or local requirements without compromising the main body of SNOMED CT. This is intended to meet the needs of different specialties and countries, regions, vendors and healthcare institutions.
Extensions are managed by Members or Affiliates who have been issued with a Namespace Identifier. A Namespace Identifier distinguishes the Identifiers of the Components created by an organization. The responsibilities of organizations that create an Extension and provide it for use by other organizations include:
The Concepts, Descriptions, Relationship and Reference Sets that form an Extension use a namespace identifier, which ensures that their SNOMED CT identifier is different from components in:
The namespace identifier is part of the component identifier. Therefore, components are distinguishable not only in the thesaurus, but also when stored in a patient record, query or decision support protocol.
Extensions use the same file structure as the core International Release. This ensures that:
Software applications should allow the users or user communities to specify the Extensions to be recognized by their systems.
An Extension should only be recognized if:
The fact that an organization is permitted to produce Extensions does not imply a seal of approval of the quality of Extensions that organization produces. Therefore a person or organization that authorizes or installs an Extension does so entirely at their own risk.
SNOMED CT has a broad clinical coverage and includes a depth of detail appropriate to a range of health care disciplines and clinical specialties. As a result, it has extensive content, different parts of which are needed in particular environments. The SNOMED CT design includes the Reference Set mechanism, which provides a standard way to refer to a set of SNOMED CT components and to add customized information to a component.
Organizations implementing SNOMED CT benefit from Reference Sets because they allow requirements for use of particular descriptions and concepts to be represented in a standard form that can be applied to any SNOMED CT enabled application. This allows Reference Sets to be shared throughout and between organizations, even when different software is used to meet local or departmental requirements.
Software developers and vendors benefit because Reference Sets provide a common, machine processable representation of requirements for different patterns of use of SNOMED CT. This simplifies local configuration and enhances interoperability with other SNOMED CT enabled applications.
Reference sets can be used for many different purposes, including:
Generic data structures for Reference Sets have been used to create a simple core structure that can be extended to meet a variety of requirements. This has been done rather than developing a complex and inextensible structure that can only be used in a finite and constrained number of ways to enforce editorial policy.
Creating a new Reference Set requires access to a namespace in order to generate SNOMED CT Ids. Within that namespace, at least one module ID concept (with an FSN and Preferred Term) should be added under the |module| sub-hierarchy (within the Core Metadata) for each of the authoring organizations. The steps required to create a new reference set include:
Please note that step 2 does not need to be performed if using one of the standard Reference Set types that have been predefined in the international release of SNOMED CT. The Reference Set Attributes for these predefined Reference Set types have already been added to the international release.
It is recommended that for each reference set, there is formal documentation that records (at a minimum) the rules, principles and approach used to determine the members of that reference set.
Reference Sets need to be maintained and the content re-examined when new releases of SNOMED CT are made available. Processes need to be established to address the concepts that have become inactive and the new concepts added in each new release.