Introduction

SNOMED CT terminology provides a common language that enables a consistent way of indexing, storing, retrieving, and aggregating clinical data across specialties and sites of care. The SNOMED CT US Edition is a combination of the full January 2026 SNOMED CT International Edition and the US Extension of SNOMED CT.

The International Health Terminology Standards Development Organisation (IHTSDO^®), trading as SNOMED International, maintains the SNOMED CT technical design, the content architecture, the SNOMED CT content (includes the concepts table, the descriptions table, the relationships table, a history table, and ICD mappings), and related technical documentation. The SNOMED CT US Edition is developed and maintained by the United States National Library of Medicine and is available to authorized Unified Medical Language System (UMLS) Metathesaurus Licensees. This material includes SNOMED CT which is used by permission of the IHTSDO. All rights reserved.

SNOMED CT was originally created by the College of American Pathologists. "SNOMED" and "SNOMED CT" are registered trademarks of IHTSDO. You must have a UMLS Metathesaurus license to access SNOMED CT US Edition data files. License information is available from NLM (https://www.nlm.nih.gov/healthit/snomedct/snomed_licensing.html). 

Background

This document provides a summarized description of the content changes included in the March 2026 release of the SNOMED Clinical Terms® US Edition.

It also includes technical notes detailing the known issues which have been identified. These are content or technical issues where the root cause is understood, and the fix has been discussed and agreed, but has yet to be implemented.

This document is available as a web resource as well as alongside the March 2026 US Edition release (https://www.nlm.nih.gov/healthit/snomedct/us_edition.html). 

Scope

This document is written for the purpose described above and is not intended to provide details of the technical specifications for SNOMED CT or encompass every change made during the release. These Release Notes should be used in combination with the SNOMED CT January 2026 International Edition Release notes. 

Content Development Activity 

Summary

The March 2026 SNOMED CT US Edition marks the seventeenth release with content authoring tooling and release production provided by SNOMED International Managed Service. Quality assurance of the release was completed by both NLM and SNOMED International. Additionally, in October 2017, NLM sunset the United States SNOMED CT Content Request System and migrated to the SNOMED International hosted and maintained US CRS (https://us-request.ihtsdotools.org). 

March 2026 US Edition of SNOMED CT Statistics 

These statistics include the active content from the January 2026 International Edition and the March 2026 US Edition.

These statistics include the content from the March 2026 US Extension.

 Quality Initiative

The Quality Initiative (QI) project is the implementation of the Quality Strategy. After a successful pilot project for the July 2018 release the next stage has been implemented for subsequent releases including March 2026.

Quality improvement tasks are being deployed to improve internal structural consistency and ensure compliance with editorial policy related to the stated modeling of content. Additionally, correction or addition of defining relationships is being carried out to accurately reflect current clinical knowledge and ensure the semantic reliability of descriptions associated with a concept. 

To see the QI improvement tasks from the January 2026 International Edition, please see the January 2026 International Edition release notes.

The following QI tasks have been undertaken and/or completed in the US Extension:

Due To Attribute Modeling Review

The US Extension contained a small number of concepts that stated “Due To” in the fully specified name but were not modeled with the Due To attribute even though a valid Due To relationship was available. The modeling for these concepts has been updated to add the Due To attribute relationship.

Metastasis from <x>

The US Extension contained a small number of concepts that contained “Metastasis from <x>” in the fully specified name. To better align with SNOMED international editorial policy and content, these concepts have been inactivated.

Fully Specified Name structure

The US Extension contained a small number of concepts where the fully specified name in the format “<x> education”. To better align with current editorial policy, the descriptions for these concepts have been updated to be “Education about <x> and the original preferred term retained as a synonym. For example, 427011000124108 |Gout education (procedure)| is now 427011000124108 |Education about gout (procedure)|. This better aligns the US Extension with the current editorial policy for Procedure Naming Conventions.

SNOMED CT derived products 

SNOMED CT to ICD-10-CM map 

The purpose of the SNOMED CT to ICD-10-CM map is to support semi-automated generation of ICD-10-CM codes from clinical data encoded in SNOMED CT for reimbursement and statistical purposes. All current pre-coordinated SNOMED CT concepts from the US Edition (US Extension + International release) within three hierarchies, (Clinical findings, Events, and Situations with Explicit Context) are potentially in scope for mapping to ICD-10-CM. 

The SNOMED CT to ICD-10-CM map is released in RF2 format within the US Edition, as well as a standalone download (https://www.nlm.nih.gov/research/umls/mapping_projects/snomedct_to_icd10cm.html). The map is also available in a human readable format (tls_Icd10cmHumanReadableMap_US1000124_YYYYMMDD.tsv) contained within a standalone download - SNOMED_CT_to_ICD-10-CM_Resources_YYYYMMDD.zip.

The SNOMED CT to ICD-10-CM map is released as Refset 6011000124106 | ICD-10-CM extended map reference set (foundation metadata concept)|. The map data are in the file xder2_iisssccRefset_ExtendedMapSnapshot_US1000124_YYYYMMDD.txt, which is in the Map folder under Refset in the Snapshot Release Type folder. 

PLEASE NOTE: Starting with the 2019 September SNOMED CT US Edition (20190301) release, the SNOMED CT to ICD-10-CM map refset data will be maintained as per the SNOMED CT RF2 specifications. 

Additional map refset information can be found in the SNOMED CT to ICD-10-CM Release Notes (doc_Icd10cmMapReleaseNotes_Current-en-US_US1000124_YYYYMMDD.pdf) found in the standalone download or online. Technical information (doc_Icd10cmMapTechnicalSpecifications_Current-en-US_US1000124_YYYYMMDD.pdf) for the map can also be found in the standalone download. 

On September 26, 2019, an updated version of the map (6011000124106 | ICD-10-CM complex map reference set (foundation metadata concept)| ) was released. This updated version included an appending of the previously released mapping refset data, originally pulled from the 20190301 release. The newly released version, which includes all of the previously released data, now includes 260,749 mapped records. This is now considered the baseline for the SNOMED CT to ICD-10-CM map. 

Route of administration

The NLM maintained Route of Administration reference set (442311000124105 | Route of administration reference set) has been in static status since 2014 September US Edition release (20140901) and was deprecated from the US Edition release package as of the September 2023 US Edition release onwards. The Route of Administration reference set is located in the file der2_Refset_Simple(Snapshot, Delta, Full)_US1000124_YYYYMMDD.txt, which is in the Content folder under Refset, in each of the three RF2 Release Type folders. The refset contains a set of terms related to the location of administration for clinical therapeutics. The purpose of the Route of Administration refset is to define a portion of which can be used in the Drug Listing section of Structured Product Labeling (SPL), or for documentation and encoding of clinical information regarding substance administrations. Only concepts that are subtypes of the concept 284009009 | Route of administration value (qualifier value) are included in the refset.

The concept "442311000124105 |Route of administration reference set (foundation metadata concept)|" + relates components (including the refsetDescriptor record) remains active, however all 144 records that comprise this refset are inactivated in the SimpleRefset file: der2_Refset_SimpleSnapshot_US1000124_20230901.txt

In order to preserve the historical audit trail within the US Edition release package, the records have been inactivated (set the "active=0") rather than being deleted from the package completely.

The Route of Administration concepts are maintained as an intensional value set in the NLM Value Set Authority Center (VSAC) (https://vsac.nlm.nih.gov/valueset/2.16.840.1.113762.1.4.1018.98/expansion). Users are required to have a UMLS account to access VSAC. If you don't have a UMLS account, you can request one here: https://uts.nlm.nih.gov/uts/signup-login

Transitive closure file

NLM provides a transitive closure script and generated transitive closure table within the SNOMED CT US Edition release. The transitive closure is the complete set of relationships between every concept and each of its super-type concepts (parents and ancestors). The transitive closure files can be used to support SQL queries when created and stored as a table in a relational database. If applied in a full release, it will contain the full history and allow for subsumption queries to be applied based on any release. The NLM transitive closure script provides a comprehensive view of all of the supertype ancestors of a concept derived by traversing all the 116680003 | is a | relationships between that concept and the root concept. The transitive closure table represents the transitive closure of the 116680003 | is a | relationships of all active concepts. NLM provides the transitive closure table in the US Edition release to provide support for easier implementation and a reference against which to check alternative algorithms. 

As of the September 2020 release, the Transitive Closure script, tls2_TranstiveClosure_US1000124_YYYYMMDD.pl, and the Transitive Closure table res2_TransitiveClosure_US1000124_YYYYMMDD.txt, are located in a separate bundled zip file (SnomedCT_TransitiveClosureResourcesRF2_PRODUCTION_YYYYMMDDT120000Z.zip) on the NLM US Edition page. For the table, the superTypeId is the id of the concept playing the supertype role, set to an Identifier of a concept. The subTypeId - Id of the concept playing the subtype role. Set to an Identifier of a concept. 

Technical Notes

Known Issues

Known Issues are content or technical issues where the root cause is understood, and the resolution has been discussed and agreed but has yet to be implemented. This can be due to a number of reasons, from lack of capacity within the current editing cycle, to the risk of impact to the stability of SNOMED CT if the fix were to be deployed at that stage in the Product lifecycle.

In addition to the Known Issues reported by the IHTSDO for SNOMED CT International Edition (SNOMED CT January 2026 International Edition Release notes), the following are Known Issues for the SNOMED CT March 2026 US Edition that have been identified and will be resolved in the next editing and production cycle: 

Resolved Issues

Resolved issues are Known Issues which were not fixed as part of the previous release lifecycle, but which have now been resolved for this March 2026 US Edition. These can be issues found during the Pre-Production or Production phases of the testing of the current release, which were resolved before the final deployment of the final production release. Finally these can be issues which were reported or found during the testing phase, but which have been closed without any action taken. 

In addition to the Resolved Issues reported by the IHTSDO for SNOMED CT International Edition (SNOMED CT January 2026 International Edition Release notes), the following Resolved Issues for the SNOMED CT March 2026 US Edition:

Technical Updates

In addition to the Technical Updates reported in the following monthly Release Notes:

...the following technical updates pertain to the US Edition:

RF2 package format

Similar to the International Edition, the US Edition follows the technical specifications for RF2 packaging format.

For future reference, the RF2 package convention dictates that it contains all relevant files, regardless of whether or not there is content to be included in each particular release. Therefore, the package contains a mixture of files which contain both header rows and content data, and also files that are intentionally left blank (including only a header record). The reason that these files are not removed from the package is to draw a clear distinction between

1 ...files that have been deprecated (and therefore removed from the package completely), due to the content no longer being relevant to RF2 in this or future releases, and 

2. ...files that just happen to contain no data in this particular release (and are therefore included in the package but left blank, with only a header record), but are still relevant to RF2, and could therefore potentially contain data in future releases.

This allows users to easily distinguish between files that have purposefully been removed or not, as otherwise if files in option 2 above were left out of the package it could be interpreted as an error, rather than an intentional lack of content in that release.

JSON file

A metadata file has been added into the US Edition package in order to provide additional information on the Release contents.  It can be found directly in the main package, and is called "release_package_information.json".

The data included so far is mostly based around additional information on the package structure and its contents, but also has data describing the language refsets included in the release, etc.

Notice of changes to the US Edition Release package

In line with the refined implementation of Annotations, we are updating all Extensions in order to align them with the updated version of the refsets in the International Edition Release package (which were first published in June 2024).

Reminder of SNOMED International plan to increase description character limit

SNOMED International, owners of SNOMED CT, propose to increase the maximum length of Fully Specified Name (FSN) and Synonym descriptions from 255 characters to 4096 characters. The requirement for this change has come from the Pharmaceutical / biologic product hierarchy, where the terming guidance for FSNs causes the current limit to be exceeded where there is a large number of ingredients in a medicinal product, which is particularly common in multivalent vaccines.

There is some likelihood that implementers will have hardcoded database schemas to 255 characters, so a significant lead time is proposed before this change would take effect. Taking into account feedback received from stakeholders, the proposed changes will be implemented in July 2026.

It is important to note that there is absolutely no intention to increase the length of a multitude of the International Descriptions. The proposal is simply to increase the maximum possible size to 4096, in order to allow a small number of necessarily longer terms to be consumed. However, most Descriptions will remain at their current length, well below 255 characters.

Please see the latest Q&A blog post by SNOMED International Technical Specialist Peter Groves Williams, who explains the proposed change, its potential benefits, the feedback process and timelines, and the issues that may need to be considered in such an update. You can also review a summary of the proposal for more details.

Feedback and suggestions

We welcome questions, comments or suggestions to improve the quality, accuracy and usability of the SNOMED CT US Edition. Please send feedback to NLM Customer Support https://support.nlm.nih.gov/.