What is the role of a National Terminology Server?

Collection: Implementation Strategy

Summary: A National Terminology Server is both a technical tool and a strategic asset. It should be the single source of truth for terminologies, support direct use by simple apps, and allow larger systems to host local copies for better performance. This allows for keeping national and local content separated, provides a public sandbox to support learning and testing, and follows a regular release schedule, ensuring everyone stays aligned.

Implementation dilemma

Facilitating access to the terminology is a key element of any implementation strategy. A National Terminology Server (NTS) is a centrally operated standards-based service that houses authoritative, version-controlled copies of SNOMED CT and other code systems, such as ICD-10 or drug dictionaries. It fulfils three complementary roles:

1. Distribution – publishes every official release, reference set, and map through an automated syndication feed, providing a better alternative to manual downloads and ensuring all stakeholders consume identical content.

2. Real-time terminology services – the NTS exposes a standard API, usually based on HL7 FHIR, using the $expand, $lookup, $validate-code, $translate and $subsumes operations so that client systems can resolve codes, value sets, and cross-maps on demand.

3. Capacity-building – provides an open sandbox, training guides, and connectathon environments that lower the barrier to entry for implementers.

Because network latency and high query volumes can affect clinical performance, most production EHRs cache the national content in a local Terminology Server instance while still subscribing to the NTS feed for incremental updates.

It is essential to consider all of these functions and requirements when defining a comprehensive National Terminology Server (NTS) strategy.

Secure terminology distribution portals, such as SNOMED International's MLDS (Member Licensing and Distribution System), play a key role in managing the licensing submission and approval process.

However, the functions of manually downloading RF2 zip files on secure portals can be significantly improved by implementing an NTS, streamlining terminology server setup, facilitating updates, and preventing version mismatches. This is why, since 2023, MLDS has incorporated terminology syndication features.

Other modern NTS platforms (e.g., NHS England’s Terminology Server, the Australian National Clinical Terminology Service, and Canada Health Infoway’s new FHIR server) also publish machine-readable releases via REST APIs or syndicated S3 buckets, enabling vendors to script and automate the ingestion process.

Additionally, an NTS can serve as a lightweight or early-stage clinical application that requires access to the terminology. For example, a public vaccination portal might validate incoming SNOMED CT codes directly against the national endpoint without hosting its own server. Meanwhile, large hospital networks typically mirror the content locally, deploying a very fast local Terminology Server that synchronizes content from the National Terminology Server.

To learn more, please reach out to implementation@snomed.org.