Approach to Value Set Constraint Specifications

5.2

5.2.1

The value set specifications are presented as tables in the following general structure:

Class Name: The Clinical Statement pattern class is identified here	Class Code: If relevant, distinct classCodes are identified here
Attribute Name: The relevant attribute(s) is/are identified here
Narrative description of vocabulary domain: The relevant narrative description of the vocabulary domain is identified here.
Value set representation: Value sets are identified here, using the SNOMED CT compositional grammar extended for the purpose of this Implementation Guide as described in Appendix B/>.
Notes: Any notes relevant to this className+classCode+attributeName value set specification are made here.

5.2.2

Specifications of the "simple" form provided in this section have some limitations but they serve two important purposes described in the following sub-sections.

5.2.2.1

A large clinical terminology, such as SNOMED CT, represents a number of lexically similar concepts which are grammatically, linguistically or semantically distinct. This phenomenon is particularly pronounced if the terminology is considered without any kind of partitioning. The coarse-grained partitioning specified by these constraints simplifies and clarifies decisions about which of a set of superficially similar SNOMED CT concepts are appropriate to particular HL7 vocabulary domains.

For example, consider a vocabulary domain specification that is intended to represent "an adverse event in reaction to a drug". The simple value set constraints in this specifications exclude these inappropriate alternatives and thus provided a helpful guide for value set developers.

• The most suitable SNOMED CT concepts to represent such an event would be those subsumed by [ <<62014003 | adverse reaction to drug ].
• However from a lexical perspective other less appropriate concepts may appear to be suitable. For example
  • The reference to "adverse drugs reaction" may suggest use of subtypes of the procedure concept [ <<396079007 | assessment of adverse drug reactions ].
  • The reference to "drug" may suggest concepts in the use of subtypes of [ <<373873005 | pharmaceutical / biologic product ] or [ <<410942007 | drug or medicament ].

5.2.2.2

The Schematic Illustrations of SNOMED CT Expressions identify the "clinical kernel" or primary clinical "focus concept" that may exist alone or as part of a contextualized expression. In most cases, the simple value set constraints in this specification apply to this clinical focus concept. In combination with the SNOMED CT concept model these constraints form a foundation for more detailed "complete" value set specifications (as explored in Detailed aspects of issues with a vocabulary specification formalism).

Simple value set constraints can be regarded as a set of subsumption clauses related by OR logic. Each clause permits the inclusion of a focus concept that is subsumed by a specified concept. In contrast, a more complete specification would check normal form transformations of candidate expressions against a variety of subsumption and role-based restrictions. In addition a complete specification require support of a full set of logical operators between clauses (i.e. OR, AND, NOT).

For example, consider a value-set constraint which indicates that the "focus concept" must be a kind of [ <<404684003 | clinical finding ].

• The concept model indicates that a [ 404684003 | clinical finding ] concept
  • can be refined by name/value pairs with attribute names such as [ 363698007 | finding site ], [ 246112005 | severity ], [ 116676008 | associated morphology ] etc.,
  • can be the value to the attribute name [ 246090004 | associated finding ]
    • as part of the definition or refinement of a [ 413350009 | finding with explicit context ]
    • as part of post-coordinated expression that includes the [ 404684003 | clinical finding ] within a context wrapper.

• A comprehensive notation for value sets that allow subtypes of [ 404684003 | clinical finding ] may therefore also need to indicate whether any limitations apply either to the refinement or situation in which the concepts are used.
  • The context wrapper may, for instance, be used to communicate negation and uncertainty and may thus be required to support some types of information. However, it may also be necessary to constrain the use of context in a manner that is relevant to the Act.moodCode or other attributes and association in the HL7 representation.

5.2.3

A simple approach to value set constraints is inevitably incomplete when applied to SNOMED CT as a result of two features supported by SNOMED CT.Both these SNOMED CT features are useful for the detailed coded capture of clinical data. However, they create a challenge for value set representation that cannot be fully met by the simple approach used in this specification. As outlined in the following sections, the inadequacy of the simple approach introduce the risks of false-positive and false-negative results.

1. The ability to create, and the requirement to communicate, post-coordinated SNOMED CT expressions.
2. the ability to use pre-coordinated expressions referring to concept that are subtypes of [ 243796009 | situation with explicit context ] [[[13]]].

5.2.3.1

Two different aspects of SNOMED CT post-coordination ("attribute refinement" and "context/situation wrapping") may result in the valid expressions being rejected by "simple" value sets tests.

Example of "attribute refinement" false negative:

The concept [ 82764005 | operation on duodenum ] could be refined by applying more specific values to its [ 260686004 | method ] and [ 363704007 | procedure site ] attributes.

If the value for [ 260686004 | method ] is refined to [ 129304002 | excision - action ] and the value for [ 363704007 | procedure site ] to [ 181247007 | entire duodenum ], the resulting expression means "excision of the entire duodenum" and we would expect this to mean the same as "total excision of duodenum". This expression would be inappropriately rejected by a "simple" value set test of the instruction [ <<173848007 | total excision of duodenum ] (i.e. "include 'total excision of duodenum' any sub-types").

• This false negative arises because [ 82764005 | operation on duodenum ] is not a subtype of [ 173848007 | total excision of duodenum ].
• In order to obtain the appropriate result, a more complex test must be performed. This involves comparison of the normal forms of the two expressions.

Example of "context/situation wrapping" false negative:

The concept [ 373573001 | clinical finding present ] can be refined by applying a more specific value to its attribute [ 246090004 | associated finding ].

If the value for [ 246090004 | associated finding ] is refined to [ 404640003 | dizziness ], the resulting post-coordinated expression means "dizziness present". This expression would be inappropriately rejected by a "simple" value set test of the instruction [ <<404640003 | dizziness ] (i.e. "include 'dizziness' and any sub-types").

• This false negative outcome is because [ 373573001 | clinical finding present ] is not a subtype of [ 404640003 | dizziness ].
• In order to obtain the appropriate result, a more complex test must be performed. This involves comparison of the normal forms of the two expressions, taking account of the default context of a SNOMED CT 'finding'.

5.2.3.2

A potential pattern of false positive value set testing would result from attempts to augment the "simple" value set specifications to include corresponding "context/situation" Concepts. The absence (by design) of an exhaustive set of "context/situation" Concepts corresponding to each "finding" or "procedure" Concept means that on many occasions only the specification of a more general "situation" Concept will guarantee that desirable Concepts will be included, but at the expense of allowing multiple false positives.

Example of pre-coordinated "situation" false positive:

Consider a value set designed to include "respiratory disorders". To avoid rejecting concepts which include explicit context, a simple value set might include [ <<413350009 | finding with explicit context ] as well as [ <<50043002 | disorder of respiratory system ]. Such a clause would include the relevant respiratory findings, including those with explicit context. However, it would also inappropriately include other findings with explicit context (i.e. non-respiratory finding with explicit context).

Failure to include [ <<413350009 | finding with explicit context ] would result in false negatives as illustrated in the previous section.

• In order to obtain the appropriate result, a more complex test must be performed. This involves restricting the inclusion of subtypes of [ 413350009 | finding with explicit context ] to those with a value for [ 246090004 | associated finding ] that are in the set specified by [ <<50043002 | disorder of respiratory system ].