The use and representation of hierarchies in
differs from the approach taken in many older code systems and classifications. This has a number of consequences that may affect the migration of queries, decision support protocols and data entry templates.
- The codes specified in ICD-9, , the and
provide information about where the code is located in a hierarchy. This allowed simple pattern matching queries to be used for some types of retrieval.
, all 'diagnoses related to the digestive system' can be retrieved by a query for all codes starting with 'D5'
SNOMED CT concept
does not provide any information about the way it relates to other
. Therefore, a simple pattern matching query cannot be used to retrieve related information represented using
. Instead, the query that specifies a
of the required
is evaluated by testing the
of the set of
Example: All 'diagnoses related to the digestive system' can be retrieved by a query for
119292006|disorder of gastrointestinal tract|
and many other code systems have a
in which each code falls within only one branch of the hierarchy. In contrast, the
is a , which means that each can by a of many different . This is a powerful feature of but it may significantly alter the results of a migrated from an earlier scheme.
Example: All 'diagnoses related to the digestive system' is some schemes may exclude codes that are primarily classified as infective disorders even they affect the digestive system. However, a
query includes all
regardless of whether they are also in another hierarchy.
- Different hierarchies:
- Hierarchies in different code systems may be based on different principles and as a result queries that are migrated to may return unexpected results. This may also relate to different interpretation of apparently identical .
Example: Should a query for the
'nephrectomy' return only patients who have had a total nephrectomy or should the results include those with a record of a partial nephrectomy? If the partial nephrectomies are to be included then how much kidney tissue must be removed to count as a nephrectomy? If the partial nephrectomies are not included then should a record entry including the
'nephrectomy' (without specifying partial or total) be included? The
determines the answer to these questions based on the defining
. The query may need to be refined to meet more specific expectations.