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The Act.uncertaintyCode is defined by HL7 as “a code indicating whether the Act statement as a whole, with its subordinate components has been asserted to be uncertain in any way.” The values of this attribute in the HL7 vocabulary are "stated with no assertion of uncertainty" (N) and "stated with uncertainty" (U).

The semantics of this attribute overlaps with SNOMED CT [ 408729009 | finding context ] values [ <<410590009 | known possible ] and its subtypes including [ 410592001 | probably present ] and [ 410593006 | probably not present]. This provides different ways to express the uncertainty of a finding and ambiguity about the impact of combining these two representations in a single Act class instance.

The following rules avoid the risk of misinterpretation by prohibiting use of the uncertaintyCode in Act class instances that are encoded using SNOMED CT. There are two sections dealing with information models which 1) contain only SNOMED content and 2) allow multiple terminologies to be used.

If an Act.code or Observation.value contains only SNOMED-CT content then the following shall apply:

  1. The uncertaintyCode attribute SHOULD be omitted from any Act instance.
  2. If necessary the uncertainty applicable SHOULD be represented as part of the SNOMED-CT expression by refining the relevant context attribute as part of a post-coordinated expression.

If an Act.code or Observation.value contains SNOMED-CT content as one permitted code system then the following shall apply:

  1. The uncertaintyCode attribute SHALL be optional in any Act instance.
  2. If the uncertaintyCode attribute is present in an Act class instance in which the Act.code or Observation.value is expressed using SNOMED-CT then:
    • The uncertaintyCode SHALL also be represented using SNOMED-CT
    • The uncertaintyCode SHALL be the same as, or a subtype of, the value of the relevant context attribute as specified in the SNOMED-CT expression
    • The uncertaintyCode SHALL be treated as equivalent to a restatement or refinement of the relevant context attribute in the SNOMED-CT expression
    • If the value of the uncertaintyCode attribute is incompatible with the above rules then this SHALL be interpreted as an error

The SNOMED CT [ 408729009 | finding context ] values provide a more specific way to express uncertainty about the presence or absence of a finding. This is therefore preferred over the use of the optional uncertaintyCode attribute.

The SNOMED CT [ 408730004 | procedure context ] does not contain values to represent "possibly done" or "probably done". As a result there is no obvious way to express uncertainty about whether a procedure has been done. This may be relevant if an informer reports something like "I think I had a tetanus vaccination but I am not sure". The current advice is to treat such information as an Observation about past history, rather than adding uncertainty value to the [ 408730004 | procedure context ] value hierarchy. However, this issue has been raised with the SNOMED Concept Model Working Group and the advice may be revised if after further consideration the [ 408730004 | procedure context ] value set is expanded.

The HL7 UVP (Uncertain Value - Probabilistic) data type was considered as this as another HL7 approach to representation of uncertainty. The UVP data type is defined as "A generic data type extension used to specify a probability expressing the information producer's belief that the given value holds." The data types specification adds that "How the probability number was arrived at is outside the scope of this specification." There is some potential for overlap as the UVP data type is a "generic data type extension". This means it can be applied to any other data type, and hence to any HL7 attribute. This data type may be applied to attribute values associated with a SNOMED CT code. For example, to express uncertainty associated with the value of a particular measurement. However, use of UVP to apply a specific level of uncertainty to a SNOMED CT concept in an Act should be avoided.

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