UK SNOMED CT Releases: A Plain-English Guide

The official NHS TRUD documentation for SNOMED CT releases is written for a specific audience that already knows what everything means. This page is for everyone else.

What you're downloading and why it's this complicated

NHS England distributes SNOMED CT as RF2 (Release Format 2) — a set of tab-separated text files. The UK release is split into editions because different organisations need different subsets, and SNOMED International requires that the International Edition be shipped separately from UK extensions.

There are effectively three things you might download from NHS TRUD:

For most purposes, the Monolith is the right choice. It contains the same clinical content as the Clinical Edition and is simpler to work with: one zip, one directory, done.

Inside the zip: Full, Snapshot, and Delta

Every UK release ships three copies of its data. They're not three versions — they're three ways of slicing the same version:

For building a terminology database, use Snapshot. Full is for audit/history purposes; Delta is for incremental updates to an existing database.

The Monolith directory tree

After extracting (uk_sct2mo_*.zip):

SnomedCT_MonolithRF2_PRODUCTION_20260311T120000Z/
└── Snapshot/
    ├── Terminology/           ← core clinical content
    │   ├── sct2_Concept_MONOSnapshot_GB_*.txt
    │   ├── sct2_Description_MONOSnapshot-en_GB_*.txt
    │   ├── sct2_Relationship_MONOSnapshot_GB_*.txt
    │   └── ...
    └── Refset/                ← extensions and cross-maps
        ├── Language/          ← preferred/acceptable terms per dialect
        │   └── der2_cRefset_LanguageMONOSnapshot-en_GB_*.txt
        ├── Map/               ← cross-maps to other terminologies (CTV3, ICD-10, etc.)
        │   ├── der2_sRefset_SimpleMapMONOSnapshot_GB_*.txt     ← CTV3 and others
        │   ├── der2_iisssciRefset_ExtendedMapMONOSnapshot_GB_*.txt  ← ICD-10 (NHS)
        │   └── ...
        ├── Content/           ← clinical groupings (e.g. care record elements)
        └── Metadata/          ← module dependency declarations etc.

The Clinical Edition zip is structurally identical, but instead of one directory it contains four: the International Edition, UK Clinical Extension, UK Clinical Refsets, and UK Edition (the combined view). Each has the same Snapshot/Terminology/ + Snapshot/Refset/ layout.

Decoding the cryptic filenames

Filenames follow a rigid naming scheme. Breaking down der2_sRefset_SimpleMapMONOSnapshot_GB_20260311.txt:

The column type codes (s, iissscc, iisssci, etc.) describe the column structure of the file — i means integer, s means string, c means component reference. They exist for tooling validation purposes and you can otherwise ignore them.

A simpler mapping:

The SimpleMap file and CTV3

The file der2_sRefset_SimpleMapMONOSnapshot_GB_*.txt is a single TSV that contains several different simple maps, identified by the refsetId column:

CTV3 is what most people are after for legacy migration. The 900000000000497000 refset contains over 524,000 mappings in the current Monolith release.

Structure of each row:

id  effectiveTime  active  moduleId  refsetId  referencedComponentId  mapTarget

• referencedComponentId — the SNOMED CT SCTID
• mapTarget — the legacy code (e.g. X200E for CTV3)
• active — filter to 1 only

What sct ndjson picks up automatically

sct ndjson scans the extracted directory and loads:

• All sct2_Concept_*Snapshot*.txt files
• All sct2_Description_*Snapshot*.txt files
• All sct2_Relationship_*Snapshot*.txt files
• All der2_cRefset_Language*Snapshot*.txt files (for preferred term selection)
• der2_sRefset_SimpleMap*Snapshot*.txt — CTV3 codes (refset 900000000000497000)

It does not currently parse ICD-10, OPCS, or other maps — those are different reference set types (ExtendedMap/ComplexMap) with more complex rule columns.

Read v2 (Read Codes)

Read v2 codes are not in current UK SNOMED CT releases. They were phased out of NHS distributions. If you need Read v2 mappings, historical crosswalk files exist but are not included in TRUD downloads.

Which TRUD item should I download?

• Item 101 — UK Clinical Edition — International + UK Clinical extension (no dm+d drugs)
• Item 105 — UK Monolith — International + UK Clinical + dm+d all-in-one

For most clinical/GP use cases, the Monolith (item 105) is recommended — it includes everything in a single zip with no layering to deal with.

Releases are roughly every 6 months (major) with minor releases approximately every 6 weeks following the SNOMED International release cycle.