# Maintenance

The ability to incorporate changes from International and National editions of SNOMED CT is key to a successful, long-term implementation. SNOMED International provides release twice a year and national release centers may provide the same or more. These updates contain important changes that may be due to changes in medical knowledge or patches on feedback from the community, and it is important that vendors are able to process these updates.

1. Ensure that the version of the given SNOMED CT edition being used within the solution is no more than 2 versions (n-2) behind the most current version. Components that may change between versions may include:
   * Concepts: new concepts added, concepts made inactive.
   * Descriptions (with terms): new terms added, terms made inactive, terms changed (minor typing mistakes may have been corrected).
   * Relationships: new relationships, relationships made inactive, additional inferred relationships from classifying the terminology.
   * Refsets: new refsets, new members to a refset, members made inactive within a refset, a refset made inactive.
2. Ensure all subsets and maps used in user interfaces, reports, queries, etc. are maintained with each new version of SNOMED CT to ensure continued clinical validity.
3. If the vendor is required to create a SNOMED CT extension, they must follow the general principles of SNOMED CT, including allocating globally unique identifiers, concept permanence (i.e. never reusing the same concept id for a different concept), and maintaining a robust audit trail for all inactivation’s (including when the code was inactivated and what codes have replaced it).

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