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Unified Medical Language System® (UMLS®)

US Extension to SNOMED CT®

Current Data Files
(expect a new version of the US Extension for each new release of SNOMED CT)

File NameRelease Date
US Extension to SNOMED CT (RF1 and RF2 versions) September 2012


Past Data Files from:





  • RF2 - current official SNOMED CT International release format as of July 2011.
  • RF1 - original official release format from 2002-January 2011.
  • NLM encourages users to transition their applications to RF2 as soon as reasonable. The IHTSDO will continue to distribute SNOMED CT in both RF1 and RF2 formats until further notice, but RF1 will be discontinued at some point in time.
  • See the SNOMED CT Technical Implementation Guide on the IHTSDO SNOMED CT Documentation page for details on RF1 and RF2 formats.  


The US Extension to SNOMED CT® is a listing of the concepts, descriptions, relationships and their history for terminology content accepted by the NLM as a formal extension to the SNOMED CT International Release.

The main purposes of the US Extension to SNOMED CT are to:

  1. Provide “rapid” access to concept IDs for use by implementers, pending action by IHTSDO on content submissions likely to be added to the SNOMED CT International Release.
  2. Provide standard terminology needed for US clinical use cases, but not generally useful in other countries, e.g., regulatory or legislatively mandated terms specific to the US.

The US Extension includes both active and inactive content that is harmonized with the most recently published version of the SNOMED CT International Release.  As the content of the extension grows and undergoes consistent review, realignment and harmonization with the International Release, users should expect changes to the US Extension related to all future releases of SNOMED CT.

The US Extension is being developed to facilitate the use of SNOMED CT as the primary coding terminology for clinical information in electronic health records, research data bases and clinical trials databases, except in the domains of medications and tests, which are covered by RxNorm and LOINC respectively.   As local vocabularies often provide variable ways of representing commonly used concepts, the use of a common list of SNOMED CT concepts will maximize data interoperability among institutions. Users unable to find terms they think are appropriate should contact the NLM to request additional content to the US Extension. Content suitable for inclusion in the International Release may be submitted by NLM to the IHTSDO contemporaneously with its evaluation, modeling and ID assignment in the US Extension. If accepted into the International Release, the corresponding US Extension entries will be linked to the International Release content and labeled as “retired” in the US Extension.

Related Documents

File description
Beginning with the March 2012 release, the US Extension to SNOMED CT is distributed in two formats:  Release Format 1 (RF1), and Release Format 2 (RF2).  Earlier versions were released in RF1 only.

US Extension releases in both RF1 and RF2 contain files that correspond to SNOMED CT files and are described in the SNOMED CT® Technical Implementation Guide, International Release. This document is part of the SNOMED CT download package available from the UMLS Terminology Services (UTS). You must have a UMLS license to access both SNOMED CT and the US Extension files. Information on how to request a UTS license is available at

The files contain only extension specific content and core content necessary to provide a bridge between the US Extension and the International Release.

Update and maintenance
A new version of the US Extension will be published approximately one month after each new release of the SNOMED CT International Release. A history file will be provided with each release of the US Extension to document the type and resolution of changes to the extension content.

SNOMED CT license requirement
SNOMED CT is owned by the International Health Terminology Standards Development Organisation (IHTSDO), of which NLM is the US Member. Use of SNOMED CT is subject to the IHTSDO Affiliate license provisions (incorporated in the License Agreement for Use of the UMLS® Metathesaurus® as Appendix 2) and is free in IHTSDO Member countries including the United States, in low income countries, and for Qualifying Research Projects in any country.

Feedback and suggestions
We welcome any questions, comments or suggestions that would improve the quality, accuracy and usability of the extension. Please send feedback via e-mail to Customer Service. Please be sure to include the subject line “SNOMED CT US Extension”.

We would like to thank and acknowledge the contributions of Julie Green and Jeff Wilcke of the Veterinary Terminology Services Laboratory at Virginia Tech, and Ian Green and Ian Arrowsmith of the United Kingdom Terminology Centre for their expertise and advice during the development of the US Extension. We also thank all past and current UMLS licensees who have submitted content for consideration to the US Extension and their continued input and data that define the extension content.