Representation of Units
The HL7 Observation.value attribute allows units to be applied to a physical quantity, range or ratio. The HL7 datatypes specification recommends the use of UCUM (Unified Code for Units of Measure [[]]) to express units in the PQ (physical quantity) datatype.
SNOMED CT contains concepts that represent most of the widely used units and these overlap with the UCUM representation. These SNOMED CT concepts could be represented in the translation sub-element of the PQ datatype. However, this would introduce redundancy and the potential for conflict between the alternative representations.
The following guidance is intended to reduce the need for redundant representation of units and maximize the opportunity for automated unit conversion.
- Wherever possible the unit element of the HL7 PQ (physical quantity) data type SHOULD be encoded using the appropriate UCUM representation and not using a SNOMED CT concept identifier [[]].
- In the case of informal units, which have no standard UCUM representation, a SNOMED CT concept identifier MAY be used in the translation sub-element of the unit element.
- Examples of informal units include "capsules" or "tablets". In these case the unit "1" (the UCUM symbol meaning "the unity") SHOULD be used and the SNOMED CT representation of the nature of the counted unit MAY then be used.
Use of UCUM representation simplifies interoperability using HL7 messages. The UCUM specification also supports translation between different types of units. It is possible to map from SNOMED CT concepts to UCUM in all cases except those where an informal unit is specified. On the other hand, since the UCUM representation is an expression syntax it can be used to represent an almost unlimited range of complex units in a formal mathematical manner. Many of the units that can potentially be represented in UCUM have no pre-coordinated equivalent in SNOMED CT. SNOMED post-coordination does not currently support the type of mathematical formalism that UCUM offers.