SNOMED CT® Release Format 1 (RF1) Guide - ihtsdo

224 | SNOMED CT Release Format 1 (RF1) Guide July 2012
Note: The discussion of rules is included although rules are not yet used in the released SNOMED CT
Cross Maps. However, this feature may be of interest to implementers.
3.2.11.5.1. Nature of the Rules
The types of rules required may vary according to the nature of the Target Scheme. However, the following
general types of rule can be recognized in several different types of mapping:
• Age or sex of the person to whom the Concept is applied.
• Qualifiers associated with the mapped Concept.
• Temporal attributes of the mapped Concept.
• Coexisting conditions:
• Including pathological conditions and other conditions (e.g. pregnancy).
• Related statements:
• Procedure applied to particular condition.
• Condition treated by particular procedure.
• Condition arising as a result of particular procedure.
• Other statements of causation (e.g. condition caused by accident).
3.2.11.5.2. Representation
Each type of rule can be expressed as a function with parameters which when evaluated returns a true or
false result (similar to the NHS Casemix grouper tools). Alternatively the rules could be expressed as SQL
style queries against an agreed common data model (similar to the NHS MIQUEST Health Query Language).
In either case, the end result is a decision that either accepts or rejects a particular mapping.
Whichever form of representation is used the Extensible Markup Language (XML) is suggested as a
general-purpose syntax because it support flexible expressions and facilitates parsing. Thus the content of
the MapRule is an XML string that contains one or more elements. According to the form of expression each
element may represent either a function with parameters or query predicate.
3.2.11.5.3. Processing order
When determining which of a set of cross map options is to be used for mapping a particular Concept a further
consideration is the order in which the rules are tested.
The following example is used to illustrate each of the options:
Concept “A” can be mapped to one or more of the five possible cross map codes (“V”,“W”,“X”,“Y”,“Z”)
• “V” applies if the patient does not have co-existing condition “B” or “C”;
• “W” applies if the patient is under aged 60 and has co-existing condition “B”;
• “X” applies if the patient is aged 60 or over and has co-existing condition “B”;
• “Y” applies if the patient is under aged 60 and has co-existing condition “C”;
• “Z” applies if the patient is aged 60 or over and has co-existing condition “C”.
In addition to the five single target code options, there are two possible combinations
• “W,Y”and“X,Z” indicating the presence of co-existing conditions “B”and“C” in either age group.
3.2.11.5.3.1. Order Implementation processing
To allow order independent processing of Cross Maps, the rules associated with all options for mapping a
Concept must be complete and mutually exclusive. For every possible occurrence of a Concept only one
Cross Map must have a mapping rule that evaluates as true.
© 2002-2012 International Health Terminology Standards Development Organisation CVR #: 30363434