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  1. FHIR Specification Feedback
  2. FHIR-39255

Implicit Value Set Definition is Confusing

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    • Icon: Change Request Change Request
    • Resolution: Persuasive
    • Icon: High High
    • FHIR Core (FHIR)
    • R5
    • Terminology Infrastructure
    • STU
    • ValueSet
    • 4.9.12
    • Hide

      Change existing text / entire section to this:

       

      Instead of a ValueSet resource as the definition, implicit value sets are defined in a specification which references the underlying code system structures and includes a prescribed URI pattern to identify the value set.  HL7 has defined implicit value set URI patterns for some key code systems.

      Implicit value sets allow the URI to serve as the basis for ValueSet operations such as $expand and $validate-code without the need to create a defining ValueSet resource instance.

      Some advantages of using implicit value sets are that they may be used:

      1. With read-only terminology servers because a ValueSet resource cannot be posted
      2. With HTTP GET requests because they are cacheable
      3. To enable text searches (google like) for concepts across the entirety of large code systems (e.g., SNOMED CT)
      4. To provide a convenient way to reference all or parts of code systems in the definition of a ValueSet or ConceptMap 

      In some cases it is not possible to express a value set definition using ValueSet.compose - but it may be possible to express the value set definition with an implicit value set URI. An example of this scenario is all SNOMED CT concept IDs that identify reference sets. Implementers SHOULD NOT create ValueSet resources where the ValueSet.url value matches the pattern of a known implicit value set. 

      Implicit value set URIs can be used anywhere a value set URI can be used. Support for implicit value set URI patterns varies across terminology servers.

      Some (but not all) code systems have implicit value set URI patterns defined by HL7 and documented for use with FHIR terminology services. Code system publishers may also define implicit value set URI patterns. FHIR terminology servers might or might not support any or all of these URI patterns. Caution should be exercised when using value set URI patterns that have not been defined by HL7 or the code system publisher. 

      Show
      Change existing text / entire section to this:   Instead of a ValueSet resource as the definition, implicit value sets are defined in a specification which references the underlying code system structures and includes a prescribed URI pattern to identify the value set .  HL7 has defined implicit value set URI patterns for some key code systems. Implicit value sets allow the URI to serve as the basis for ValueSet operations such as $expand and $validate-code without the need to create a defining ValueSet resource instance. Some advantages of using implicit value sets are that they may be used: With read-only terminology servers  because a ValueSet resource cannot be posted With HTTP GET requests because they are cacheable To enable text searches (google like) for concepts across the entirety of large code systems (e.g., SNOMED CT) To provide a convenient way to reference all or parts of code systems in the definition of a ValueSet or ConceptMap  In some cases it is not possible to express a value set definition using ValueSet.compose - but it may be possible to express the value set definition with an implicit value set URI. An example of this scenario is all SNOMED CT concept IDs that identify reference sets. Implementers SHOULD NOT create ValueSet resources where the ValueSet.url value matches the pattern of a known implicit value set.  Implicit value set URIs can be used anywhere a value set URI can be used. Support for implicit value set URI patterns varies across terminology servers. Some (but not all) code systems have implicit value set URI patterns defined by HL7 and documented for use with FHIR terminology services. Code system publishers may also define implicit value set URI patterns. FHIR terminology servers might or might not support any or all of these URI patterns. Caution should be exercised when using value set URI patterns that have not been defined by HL7 or the code system publisher. 
    • Reuben Daniels/Rob Hausam: 4-0-0
    • Clarification
    • Non-substantive
    • R5

    Description

      The existing text: 

      Implicit value sets are those whose specification can be predicted based on the grammar of the underlying code system, and the known structure of the URL that identifies them. The implicit value set capability allows a single URL to serve as a value set definition, and can serve as the basis for the $expand operation and for other value set references. If any value set resources exist with an identifier that conforms to the specified implicit value set URL patterns, the content of the resource must conform to the template provided. Profiles and other value set references can reference these value sets directly (by reference as a URI, rather than by a literal value set reference).

      Some (but not all) code systems have implicit value set URL patterns that have been defined and documented for use with FHIR terminology services. Particular FHIR terminology servers might or might not support any or all of these URL patterns.

      ********

      This material is not clear about:

      1. What the "known structure of the URL that identifies them" means. Does it mean the known structure of the canonical url for an implicit value set?

      2. Is there a CLD for an implicit ValueSet? Is there a resource instance? 

      3. Suggest creating an example of an implicit value set. 

      4. The text should address how one knows the template for a value set url pattern and how one would know the specified implicit value set URL pattern. If those are defined by the underlying code system, that information should be included in the text. 

      5. Clarify whether an all codes value set an implicit value set.  The Using CPT with HL7 Standards section on implicit value sets lists the all codes value set. That is very different than the information for UCUM for example, and muddies the picture.

      After reading this section, and following all the links (RxNorm does not define any implicit value sets), I still don't know what an implicit value set is. 

       

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            carmela_couderc Carmela A. Couderc
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