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Current Version - Under Revision

Moving from a legacy coding scheme to SNOMED CT requires attention to be paid to continued accessibility and use of data encoded using the legacy scheme.

The following general approaches may be applied or adapted taking accounts of the capability of the SNOMED CT enabled application and the value and relevance of existing data.

  • Mapping or converting the data:
    • This requires each instance of a code in the existing data to be mapped to an appropriate

      SNOMED CT expression

      . This


      is then associated with the existing coded record entry as part of a record entry that conforms to the information model used in the

      SNOMED CT enabled application

  • Linking or integrating existing data:
    • The existing data is retained in its native form or in an intermediate form. However, the

      SNOMED CT enabled application

      is designed (or adapted) to access this existing information as if it had been converted. To deliver this functionality, queries and/or data are in effect mapped at the time of retrieval, rather than at the time of upgrading the system. This can be achieved in different ways some of which involve direct use of mapping tables and others in which, while the existing data is unchanged, an index derived from a map to


      is generated to optimize subsequent access.
  • Archive or retain old data in its original form, and where it is necessary to retrieve historical information, use


    from the legacy system to do this:
    • This approach completely separates the new


      from the legacy data and is unlikely to be acceptable in clinical practice . However, it may be appropriate for some data warehousing applications where the wholesale conversion of data is considered too onerous.