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Representing Compound Statements and Relationships between Statements

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In the HL7 clinical statement model the ActRelationship class is used to express links or associations between different clinical statements. These linkages may be of different types expressed using the typeCode attribute. Examples of typeCode values include "contains", "pertain to", "caused by", and "reason for".

SNOMED CT provides a variety of attributes that can be used to represent relationships between different concepts in a post-coordinated expression. These post-coordinated expressions have the potential to represent some association that might alternatively be represented using ActRelationships. For example, the attributes [ 42752001 | due to ] could be used to construct expressions such as [ 49218002 | hip pain |: 42752001 | due to | = 396275006 | osteoarthritis ], which could also be represented using two separate Observations linked by an ActRelationship with the typeCode "caused by".

There is no absolute rule about when to express linkage in the terminology and when to use linkage mechanisms in the RIM (e.g. ActRelationships). However, the following guidance should be followed:

  • A single identifiable observation, assertion or procedure SHOULD usually be represented by a single Act class instance containing an appropriate SNOMED CT expression.
  • A single Act class instance SHOULD be used to represent an integral combination of facets applicable to a single identifiably observation, assertion or procedure. Some examples of integral combination are shown below. The common feature of these is that together they represent a finding with a distinct pattern and a shared life history.
    • A combination of findings is a part of a single recognizable condition
      • E.g. "Headache preceded by visual disturbance".
    • A disorder is specialized by a specific cause
      • E.g. "Pneumonia due to streptococcus pneumoniae".
    • The nature of a disorder is determined by another condition
      • E.g. [ 4855003 | diabetic retinopathy ].
    • A temporal or causative relationship between two concepts in which one is a specific symptom or diagnostic criterion for the other.
      • E.g. [ 51771007 | postviral fatigue syndrome ], "Shortness of breath after moderate exercise".
    • A single recognized procedure involves two or more distinct but related actions:
      • E.g. [86477000 | total hysterectomy with removal of both tubes and ovaries ], "Reduction and fixation of a fracture"
  • Post-coordinated SNOMED CT expressions SHOULD NOT be used to artificially combine distinct observations, assertions and procedures into a single Act class instance.
    • The line between integral combinations of items and distinct items is not clear-cut. However, as a general rule two items SHOULD be considered to be distinct if
      • they are capable of being independently validated (i.e. the accuracy of one statement is not dependent on the accuracy of the other)
      • their life histories differ and are independent of one another
      • the relationship between them is a matter of judgment rather than fact
  • Distinct observations, assertions and procedures SHOULD be represented by separate Act class instances related to one another by appropriate ActRelationships.
    • Multiple distinct findings in a patient that may or may not be associated with one another or with some more general problem.
      • E.g. A collection such as "chest pain" with "shortness of breath" finding of "tachycardia" and "ECG abnormality" interpreted as "Myocardial infarction".
    • Multiple conditions occur contemporaneously (or in sequence) where the nature of individual conditions is specific to the presence of the other condition.
      • E.g. "AIDS" and "gastro-enteritis"
    • Multiple distinct procedures incidentally performed at the same time or during the same hospital stay.

In general SNOMED CT expressions (whether pre-coordinated or post-coordinated) are most appropriate for expressing multiple facets of a single logical concept. On the other hand, HL7 ActRelationships are more appropriate for making associations between multiple distinct observations or procedures. However, this boundary is fuzzy and there are many situations in which either approach may have equal merit.

The use of SNOMED CT attributes may result in arbitrarily complex statements that wrap multiple distinct findings within a single terminological expression. In these cases, the use of separate coded statements linked by Act Relationships is preferable. On the other hand, use of multiple statements linked by ActRelationships to represent a single composite finding or procedure may result in loss of the natural clinical term used by a clinician within a collection or linked classes.

Even when the guidelines above are followed, there will be grey areas. In an ideal world rule would be devised to compute equivalence between single Act class instances containing a post-coordinated SNOMED CT expressions and multiple Act class instances. While this is theoretically possible, there are several practical obstacles. The HL7 vocabulary for the ActRelationship.typeCode attribute differs from the range of values for linkage attributes in SNOMED CT. Simple precise or close mappings exist for some values but more work is needed before we can assert full semantic interoperability between the two representations. In addition, while a single instance post-coordinated representation has a single life-history the individual instances in multiple class representation may have separate life histories and separate associations with other contextual information.