The Language of Healthcare: Exploring US Extension of SNOMED CT
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Welcome back to our blog series on essential healthcare terminologies that are transforming data management and patient care. So far, we’ve covered SNOMED CT - a comprehensive standard for clinical terminology, ICD-10-CM - a vital classification system for coding health conditions and procedures and LOINC - a universal standard for identifying health measurements, observations, and documents. This week, we will explore SNOMED CT a bit further, investigating the usage of extensions within SNOMED CT.
In this post, we will use the US Extension of SNOMED CT as our example. We will go over the need for having the US Extension, compare it to the SNOMED Core, and understand its governance.
Don’t forget to explore TermHub’s public US Extension of SNOMED CT project to get hands-on experience with this extension. Stay with us throughout this series as we continue to unpack the healthcare terminologies that matter most. Follow TermHub on LinkedIn to get timely insights that can help you stay at the forefront of healthcare data management.
What is a SNOMED CT Extension?
A SNOMED CT Extension is an officially governed expansion of the international SNOMED CT terminology that allows countries, regions, or specialized organizations to add locally relevant clinical concepts while remaining fully interoperable with the global SNOMED CT Core.
Rather than modifying the international standard, an extension adds new concepts, descriptions, and relationships using the same SNOMED CT data model. Because extensions follow the identical structure and editorial rules defined by SNOMED International, systems that already support SNOMED CT Core can incorporate extension content without breaking interoperability.
What defines a SNOMED CT Extension?
Below are the key points that define a SNOMED CT Extension:
1. It is a formal SNOMED International construct
Extensions are developed under SNOMED International governance rules. They follow the same quality standards, release processes, and technical specifications as the core terminology.
2. It uses the identical SNOMED CT data model
Extensions rely on the same core components:
Concepts
Descriptions
Relationships
Historical tracking mechanisms
This ensures that extension content behaves exactly like core content within terminology services, EHRs, and analytics platforms.
3. It is distinguished by a unique moduleId
Every extension component is assigned a specific moduleId, which identifies its source and governance authority. This allows systems to clearly differentiate between:
SNOMED CT Core concepts
National extensions
Regional or organizational extensions
The moduleId is visible in RF2 release files, APIs, and terminology browsers, making provenance and ownership transparent.
US Extension concept with Module Id highlighted. See how the module id is defining the concept’s owner as US Extension rather than SNOMED Core.
4. Extensions can be layered
Organizations can build extensions on top of other extensions. For example:
The SNOMED CT US Extension builds on SNOMED CT Core
U.S.-based organizations such as the Veterans Health Administration or Kaiser Permanente may build additional local extensions on top of the US Extension
This layered model allows local customization while preserving compatibility with the international standard.
Extensions vs. Editions
It is important to distinguish between an Extension and an Edition:
A SNOMED CT Extension contains only the additional content governed by that extension’s moduleId.
A SNOMED CT Edition bundles the SNOMED CT Core together with one or more extensions to create a complete, distributable release.
For example:
The US Extension of SNOMED CT includes only U.S.-specific content.
The US Edition of SNOMED CT includes both the international Core and the US Extension bundled together.
Why SNOMED CT Extensions Matter
SNOMED CT Extensions enable national and organizational customization without sacrificing global interoperability. They allow healthcare systems to:
Represent country-specific clinical requirements
Meet regulatory or reporting mandates
Support localized workflows
Maintain compatibility with international SNOMED CT implementations
In short, a SNOMED CT Extension is a structured, governed way to localize clinical terminology while preserving semantic interoperability at scale.
Why do I need a US Extension to SNOMED CT?
The SNOMED CT US Extension is essential because the content found in the SNOMED CT International Edition (SNOMED Core) may not fully address the specific clinical, administrative, and regulatory requirements unique to the United States.
Here are the key reasons for its existence:
Addressing US Specific Clinical Needs
The International Edition might lack terms or concepts frequently used in US healthcare settings, such as terminology for localized conditions, treatments, or protocols. However, US healthcare organizations and systems must exchange this data seamlessly across diverse settings. The US Extension supports this effort by including content that aligns with local clinical practices, ensuring improved data exchange and interoperability within the US.
Supporting US Standards and Reporting
The US healthcare system relies on specific coding systems like ICD-10-CM, CPT, and LOINC for billing, reporting, and regulatory compliance. The US Extension provides mappings to these standards, ensuring SNOMED CT can integrate seamlessly into US healthcare workflows.
Reflecting Regional Language and Preferences
Clinical language can vary by country and even regionally within the US. The US Extension adds terms and synonyms that reflect US clinical language, making SNOMED CT more practical and user-friendly for US clinical providers.
What type of content is contained within the SNOMED CT US Extension?
The SNOMED CT US Extension contains content tailored to meet the unique requirements of healthcare delivery and reporting in the United States.
This includes:
Additional Concepts
Concepts not present in the International Edition but needed for US healthcare, such as specific clinical conditions, procedures, or terminologies. Some of these may eventually be moved to the International Edition in future releases. The extension also includes US-specific clinical or administrative concepts.
Descriptions
US-specific synonyms or preferred terms for existing international concepts as well as alternate descriptions to reflect common language or usage in US clinical practice. This helps ensure SNOMED CT is more intuitive and user-friendly for healthcare providers in the US.
Relationships
Modifications or additions to relationships between concepts to support US use cases. This enables a more accurate and relevant representation of medical knowledge within the US clinical context.
Mappings
Links to US-specific standards, such as ICD-10-CM, CPT (Current Procedural Terminology), and other coding systems for billing, reporting, or analytics. These mappings facilitate the integration of the US Extension of SNOMED CT with other widely-used coding systems in the US.
Reference Sets
Reference sets specific for SNOMED CT use within the US. These include predefined groups of concepts that meet specific US clinical and administrative needs.
Who maintains the SNOMED CT US extension?
The SNOMED CT US Extension is maintained by the National Library of Medicine (NLM), a division of the National Institutes of Health (NIH). The NLM oversees the United States-specific modifications and extensions to the SNOMED CT standard. The NLM ensures it aligns with national healthcare requirements and standards, and that it meets the unique healthcare requirements, regulatory frameworks, and clinical practices in the United States. This includes ensuring that US healthcare systems can use SNOMED CT effectively in ways that align with both national standards and local practices.
The NLM provides access to the SNOMED CT US extension via its Unified Medical Language System (UMLS) Metathesaurus, which integrates a wide array of biomedical terminologies and standards. This enables interoperability across a broad range of healthcare information systems, ensuring that SNOMED CT can be used in conjunction with other essential coding systems and terminologies that support clinical decision-making, administrative reporting, and regulatory compliance.
How often is the SNOMED CT US Extension updated?
The SNOMED CT US Extension is updated biannually, with releases typically occurring in March and September.
These updates align with the old biannual release cycle of the SNOMED CT International Edition, which prior to being released monthly, was updated twice a year (in January and July).
The updates incorporate:
New or updated content from the International Edition, including revised terminology or concepts that have been globally accepted.
Additions or modifications specific to US healthcare requirements, such as new concepts, descriptions, and mappings for emerging diseases, medical procedures, and US regulatory requirements.
Ongoing maintenance to ensure continued compatibility with evolving US healthcare standards, regulatory frameworks, andclinical practice guidelines..
The updated content is made available through the Unified Medical Language System (UMLS), authorized distribution channels such as the National Library of Medicine’s website, and other licensed data providers.
How do I request content in the SNOMED CT US Extension?
To request new content or modifications in the SNOMED CT US Extension, you can follow these steps:
Identify Your Need
Clearly define the concept, term, or relationship you want added or modified and ensure the requested content is not already available in the SNOMED CT International Edition or US Extension. For batch requests, you will need to use a special template that you can download here.
Submit a Request
Use the US SNOMED CT Content Request Service (USCRS) provided by the SNOMED International. Access via the website: https://us-request.ihtsdotools.org/.
Provide Detailed Information
Include details such as:
Description(s) of the concept or term.
Its intended use (e.g., clinical documentation, reporting, billing).
Supporting references or evidence, such as clinical guidelines or standards.