The permissible value for the SCTIDs are limited by the following rules:
The second and third digits from the right hand end of the string rendering of the identifier must match one of the partition-identifier values specified in this guide.
The rules also ensure that an SCTID can be distinguished from code from one of the antecedent code systems Read Codes(which are 4 or 5 characters in length) and legacy Identifiers from SNOMED RT and it predecessors (which always start with a letter).
SNOMED RT identifiers are SCTIDs identical to those used in SNOMED CT but in some cases will now refer to inactive concepts. In these cases, data in the 900000000000489007 |Concept inactivation indicator reference set|and 900000000000522004 |Historical association reference set|can be used to find the identifier of the closest equivalent active concept.