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Cross mapping

What it is

Cross-mapping is the link between SNOMED CT and the Read codes to the mandated versions of the NHS Clinical Classifications National Standards ICD-10 (diagnosis) and OPCS-4 (intervention and procedures).

The NHS Classifications Service maintains the maps. These are updated twice a year to ensure cross-maps exist where appropriate for new terminology content, as well as timely availability of content and cross-maps for revisions to the classification.

Terminologies used in the NHS which have supporting classification cross-maps are:

  • SNOMED Clinical Terms  (SNOMED CT)
  • Clinical Terms Version 3 (The Read Codes) (CTV3)
  • The Read Codes Version 2   (V2)

Cross-mapping is carried out in one direction from source SNOMED CT concepts and the Read codes to target ICD-10 and OPCS-4 codes.


Originally the cross-maps were developed to ensure that organisations using systems incorporating the terminologies were able to fulfil the mandated requirement for the collection and reporting of Admitted Patient Care data. This is a continuing requirement with the terminology to classification cross-maps supporting functions such as:

    • Epidemiology
    • Clinical Governance
    • Commissioning
    • Payment by Results
    • Public Health
    • Clinical Audit
    • Performance improvement.

More recently the classifications ICD-10, and particularly OPCS-4, provide the underlying data for Payment by Results (PbR) which is the Department of Health data driven funding system for the NHS in England.

How the cross-mapping scheme works

The scheme employs electronic tools and a methodology which use a series of flags to apply UK ICD-10 and OPCS-4 standards, rules and conventions within the terminologies. For example:

    • Dagger and asterisk system
    • Sequencing rules
    • Conventions such as 'See also'.

The mappings are semi-automatic, with default and alternative codes, where appropriate. This allows the coding professional to include context, if available in the clinical record, in the final target code(s) assigned.

Drivers for Change

The drivers for additional and changed cross-maps are many and include:

    • World Health Organisation updates to ICD-10
    • SNOMED CT changes including new core content
    • Revisions of OPCS-4 (as part of the annual review cycle)
    • Requests from the NHS (via the helpdesk)
    • Output from the UK Coding review Panel
    • Changes in support of Department of Health initiatives (including PbR).

Why the classification cross-maps are important

The link between the terminologies and the classifications is essential to support the derivation of management data from clinical information systems and is fundamental to the production of statistical data used by the Department of Health to produce Hospital Episode Statistics (HES) and support the Secondary Uses Service (SUS). The data supports functions such as:

The provision of national clinical coding standards via the cross-maps (PDF, 129.5kB) for users of The Read Codes and SNOMED CT is pivotal for data submitted to SUS and PbR.

Other Uses of the Cross-maps

NHS Connecting for Health maintain and develop mappings from the National interim Clinical Imaging Procedure (NICIP) codes to SNOMED CT concepts which in turn are cross-mapped to OPCS-4.6. A preliminary mapping table from SNOMED CT diagnostic Imaging codes to OPCS-4.6 is available from the Data Standards and Products electronic distributions service (TRUD).

Data Files and licensing

The cross-map data files are included within the UK Terminology Centre (UKTC) releases and are available for download (following registration and completion of license agreement) from the Technology Reference Data Update Distribution Service (TRUD).


All queries and requests regarding the classification cross-maps should be submitted using the cross mapping query form to the National Clinical Classifications helpdesk at:



SNOMED CT is the chosen terminology for the use in the NHS in England. For those classification users wishing to gain a basic understanding of SNOMED CT there is a SNOMED CT e-learning course available on the Data Standards website. Additionally there is a short leaflet available for download which briefly describes SNOMED CT and the benefits.