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The SNOMED CT US Extension (the "extension") is maintained by the National Library of Medicine, part of the National Institutes of Health, Department of Health and Human Services. The US Extension consists of US created components that reside in the US Extension.

The US Extension is made up of two different types of content, US-specific content and content of US origin that is intended for inclusion in the International Release. Extension content may be placed in three different categories:

  1. Permanent US Extension content – Content that has been assigned US Extension identifiers for use in US specific applications. This content is not intended for consideration as a component of the International Release.
  2. Long-term US Extension content – Content that has been submitted with the intention to add to the International Release of SNOMED CT, but due to unresolved international editorial policy issues or lack of international use cases, is "temporarily" added to the US extension pending resolution of the issues. Upon promotion of this content to the International release, the US extension identifiers assigned to Concepts and Descriptions will be retained, unless significant remodeling by the International editors is needed to conform to International editorial policy. 
  3. Short-term US Extension content – Content included in the US Extension are assigned extension component identifiers to provide immediate access to the terms for use in applications. These terms have been submitted for inclusion in the International Release and the US extension identifiers for promoted concepts will be retained, unless significant remodeling by the International editors is needed to conform to International editorial policy. 

Rationale for the NLM SNOMED CT US Extension

The IHTSDO "enables authorized organizations to add Concepts, Descriptions, Relationships and Subsets to complement the core content of the SNOMED CT International Release.  One example of the Extension mechanism is extensibility of SNOMED CT for the specialized terminology needs of an organization \[or member country\]." (SNOMED Clinical Terms® User Guide January 2010 International Release). These extension components are globally uniquely identified, but retain the common SNOMED CT structure for full integration into the International Release. The National Library of Medicine develops and maintains a set of SNOMED CT components for the general use of US Affiliates. The terms originate from requests submitted by US licensees that are approved by the NLM and placed into one of the three categories listed above. 

Purpose of the US Extension

The SNOMED CT US Extension addresses a multitude of current needs that cannot be immediately addressed by the content of the SNOMED CT International Release (the "core"). Specifically the extension provides:

  1. Concepts that are jurisdictionally or domain specific.
  2. "Rapid" access to concept IDs for use by implementers.
  3. Interim concept IDs for implementers pending addition to the SNOMED International Edition.
  4. Regulatory or legislatively mandated terms specific to the US.
  5. Domain content external to the current scope of the SNOMED CT International Edition.

Creating extension components

Extension components are created in a manner identical to the process for creating SNOMED CT core content. A policy describing the allowable changes that may be made within an extension is available at: SNOMED International response to "Discussion paper - Allowance of Extensions to Modify Core Content" - Version 2.1. That is, content is evaluated as to its appropriateness for inclusion and is then semantically and structurally modeled to fit within one of the existing top level hierarchies of the core or into an extension specific top level hierarchy. All concepts have assigned parents (directly or transitively) that are existing active concept(s) in the core. Extension terms may be distinguished from core content by the moduleID to which they are assigned. All extension components contain a namespace identifier, assigned by SNOMED International, which uniquely identifies the origin of the extension content.  Additionally, all extension components have a partition assignment that identifies the component as belonging to an extension. For example:

Figure 1. SCTID Structure for a component in an Extension

The National Library of Medicine's Namespace Identifier for the US Extension is: 1000124. Extension components are also assigned a ModuleID that indicates the organization that "owns" the component. This moduleID is modified when a component is moved from an extension to an ancestor extension or the core.

Sources of extension content

Potential US Extension content comes from multiple sources. A primary source of content is existing content stored in local/organizational extensions. These extensions are developed and maintained within approved namespaces independently from other approved namespaces and may contain duplicate content among them. A critical issue in the evaluation of external extension content is the high degree of variability in the quantity and quality of attribute modeling that accompanies extension concepts. 

A second source of potential extension content comes from Affiliate individuals and organizations that do not possess an extension namespace, but currently provide request submissions directly to the US SNOMED CT Content Request System (US CRS). Additional information on how to submit requests for additions and changes to the terminology is described in the subsequent sections. 

As the US National Release Center (NRC) for SNOMED CT, the NLM has the responsibility for transmitting content requests to SNOMED International. Through the editing and vetting process, the NLM terminology editor may forward an Affiliate's request to SNOMED International for consideration.  SNOMED International then has the right to accept or reject the request, which then would be passed back to NLM for consideration as US Extension content. 

A third source of content may be independently developed terminologies, where the NLM works directly with the standard development organization (SDO) under an agreement to integrate content into the US Extension.

A final source of content is that determined by the US extension content developers to be required for completeness of the terminology to support clinical recording or analytics.  This content is usually identified through a gap analysis of the terminology during editing of requests that originate from other sources.

Submission to the US Extension

The U.S. SNOMED CT Content Request System (US CRS) is available for users to request for new content or modifications to the SNOMED CT terminology. A SNOMED International Confluence account to access the tool. A form to request access is located here: Confluence User Accounts. Under 'Why do you need a Confluence account, indicate that it is to submit USCRS requests and click the appropriate box in the next section. For issues related to creating a login, please contact the NLM SNOMED customer support.

Once logged in through IMS, users are automatically logged into the US CRS. Choose the USCRS from the list of IHTSDO tools to be redirected to the US CRS dashboard. See the US CRS User Guide for additional documentation on how to use the tool.

There are three methods for creating SNOMED CT content erequests: Simple Mode,  Direct Mode, and Batch Mode. To learn more about these modes and help choosing the correct one see: Creating Requests

An understanding of SNOMED CT content and structure is required to create successful requests. Each request must include a SNOMED CT identifier that links to an existing concept in either SNOMED CT or the US Extension. A SNOMED CT browser is available here, which allows free browsing of the International release and a number of national editions, including the US edition. Every request must also provide a definition, justification or practical use case. Authoritative references describing the clinical applicability of the concept is strongly recommended. See the NLM Learning Resources Database for tutorials on using US CRS. 

Request management

When received by the NLM, an acknowledgement email will be sent stating that the request has reviewed by the US content editor at the NLM. As the request proceeds through the request management process, emails will be sent informing the requester with the date and type of status change. Following review of a request, email notifications of inquiries for additional clarification or answers to specific questions concerning the request may be sent. When a request has been completely fulfilled (i.e. added to the US extension, added to the SNOMED CT International release or not accepted for inclusion), a final status email will be sent to the submitter.

Disposition of a request falls into one of the following categories: 

  1. Request accepted and forwarded to IHTSDO for inclusion in a future release of SNOMED CT.
  2. Request accepted and the changes or additions reflected in the US Extension.
  3. Request rejected (with explanation).
  4. Request placed on hold for future consideration.

During the course of the review of submitted content, editorial changes to the submission that clarify the meaning or conform to SNOMED CT modeling rules may be made to the requested items. Every attempt is made to retain the original meaning of the submitter. If appropriate content cannot be constructed that meets the submitter needs, the content is placed in a CLARIFICATION REQUESTED status for further discussions to resolve outstanding issues. At the end of the process, unless rejected, requested terminology should appear in either the next International or US Edition release of SNOMED CT following the completion date of the request.

Further information regarding the US CRS application, process, and Service Level Agreement can be found on the US CRS User Guide

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