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Procedure.approachSiteCode and SubstanceAdministration.approachSiteCode

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The Procedure.approachSiteCode is defined by HL7 as "the anatomical site or system through which the procedure reaches its target (see targetSiteCode)."

SNOMED CT procedure concepts have a defining attribute that specifies the [ 424876005 | approach ] and has a comparable meaning. The post-coordination rules that apply to SNOMED CT permit refinement of this defining attribute. The resulting post-coordinated expressions can be represented in a single coded attribute using the HL7 Concept Description (CD) data type.

The result of this is that there are two completely overlapping approaches to the representation of approaches associated with procedures.

While HL7 models SubstanceAdministration as a separate class from Procedure, the SNOMED CT concept [ 432102000 | administration of therapeutic substance ] is a subtype of procedure. Therefore the [ 424876005 | approach ] attribute can also be applied to refine SNOMED expressions that encode the action associated with SubstanceAdministration. Therefore, this overlap also applies to that class.

The following rules avoid redundancy and the risk of misinterpretation by restricting the use of the approachSiteCode in Procedure and SubstanceAdministration class instances. There are two sections dealing with information models which 1) contain only SNOMED content and 2) allow multiple terminologies to be used.

If a Procedure.code or SubstanceAdministration.code contains only SNOMED-CT content then the following shall apply:

  1. The approachSiteCode attribute SHOULD BE omitted from any Act instance.
  2. If necessary the specific site applicable SHOULD be represented as part of the SNOMED-CT expression (in Procedure.code or SubstanceAdministration.code) by refining the relevant site attribute as part of a pre or post-coordinated expression.

If a Procedure.code or SubstanceAdministration.code contains SNOMED-CT content as one permitted code system then the following shall apply:

  1. The approachSiteCode SHALL be optional in any Act instance.
    • The approachSiteCode SHALL also be represented using SNOMED-CT
    • The approachSiteCode SHALL be the same as, or a subtype of, the value of the relevant site attribute as specified in the SNOMED-CT expression
    • The approachSiteCode SHALL be treated as equivalent to a restatement or refinement of the relevant site attribute in the SNOMED-CT expression
    • If the value of the approachSiteCode attribute is incompatible with the above rules then this SHALL be interpreted as an error

The notes following the Procedure.approachSiteCode definition suggest that the intent is not to repeat the approach if it is fixed by the nature of the procedure specified by the Act.code.

Some [ 424876005 | approach ] sites can also be "pre-coordinated" in the Act definition, so that there is never an option

to select different body sites. The same information structure can handle both the pre-coordinated and the post-coordinated approach.

Therefore, if the Procedure.code or SubstanceAdministration.code specifies the [ 424876005 | approach ] to a sufficient level of detail, there is no requirement to include a separate approachSiteCode attribute. When using SNOMED CT post-coordination to refine the [ 424876005 | approach ] , the Act.code can specify the approach to the same level of detail as can be achieved using the approachSiteCode attribute.

The vocabulary domain specified for approachSiteCode is ActSite which is the same as the vocabulary domain for targetSiteCode. In contrast SNOMED CT uses a specific value hierarchy for approaches which is different from the one used for [ 363698007 | finding site ] or [ <<363704007 | procedure site ]. The distinction is that an approach is a route used to reach a target site rather than a specific structural landmark that represents a point on or part of that route.

The example values in the approachSiteCode include a mixture of approaches (e.g. "trans-abdominal approach" and "retroperitoneal approach") which fit the idea of approach as used by SNOMED CT. However, references to the punctured area of skin or structural landmarks have a significantly different semantic quality. Many sites are never the names of routes, several routes may pass through a single site and a route may pass through several sites. Therefore attempts to combine SNOMED CT and HL7 representations of approach may result in confusion rather than clarity.

The recommendation to use the SNOMED CT representation of [ 424876005 | approach ] is based on the more appropriate range of values available for this attribute, and on the fact that many procedure concepts pre-coordinate an implied or explicitly stated approach. Omission of the HL7 approachSiteCode attribute is recommended to avoid the redundancy and the potential for conflicts between the two forms of representation of site. However, while the use of these HL7 attributes is deprecated, it is permitted to support use in environments that do not support SNOMED CT post-coordination. In this case, requiring the approachSiteCode attributes to be encoded using SNOMED CT and interpreted as refinements of the SNOMED CT [ 424876005 | approach ] , enables a simple transformation to the recommended form.