Representing Units of Measure (For Use with Numerical References and Values)

Printer Friendly, PDF & Email
Type Standard Implementation/Specification Standards Process Maturity Implementation Maturity Adoption Level Federally required Cost Test Tool Availability
Rating 3
Limitations, Dependencies, and Preconditions for Consideration Applicable Value Set(s) and Starter Set(s)
  • UCUM is a syntax for representing units of measure for use with numerical references and values. It is not an enumerated set of codes.
  • The case sensitive version is the correct unit string to be used for interoperability purposes.
  • Per public comments received, there may be some limitations with UCUM in the laboratory domain that remain unresolved.
  • The abbreviations used for a few of the units of measure listed in the UCUM standard are currently on lists of prohibited abbreviations from the Institute for Safe Medication Practice (ISMP).
  • Some abbreviations for units of measure include symbols which may be in conflict with other HL7 standards.
  • Some abbreviations for units are nonstandard for human understanding.(For example, if a result for a White Blood Cell count is 9.6 x 103/μL, the UCUM recommendation for rendering this value in a legacy character application is 9.6 x 10*3/uL. Because the “*” is a symbol for multiplication in some systems.) This recommendation may result in errors either by the information system or the human reading the result.
  • Some abbreviations used in UCUM are not industry standard for the tests that use these units of measure.
  • Units Of Measure Case Sensitive 2.16.840.1.113883.1.11.12839 (most frequently used codes)
  • “Table of Example UCUM Codes for Electronic Messaging" published by the Regenstrief Institute, Inc. Value set is made available at and identified by the OID



UCUM validators exist. Ability to test if unit is valid or unit1 converts to unit2 are all valid test tasks. Consider changing n/a in testing to linking to validation and conversion tools.





Table 3: Dimensionless units. The units ppb and ppt are deprecated because the names “billion” and “trillion” are ambiguous. The expression “10*-9” or “10*-12” should be used instead. When the units percent or “parts per N” are used for concentrations specific units are prefered, e.g., “ug/l” for mass concentration. The expression “ug/kg” for ppb is also valid.
the number ten for arbitrary powers  10n  10*  10*  no 10  1 
the number ten for arbitrary powers  10n  10^  10^  no 10  1 


Both * and ^ can be used for arbitrary powers as shown in the table above (see

Recognize a value set that uses the ^ form.

WHICH values in HL7 conflict? I know SOME values are different, but I have seen none that conflic. See




We strongly concur with the listing of UCUM here. Daniel Vreeman has compiled a list of UCUM conversion and validation resources here. It is worth noting that UCUM is designed for accomplishing a computable units representation in electronic communication. This is the main interoperability need. Having display strings for humans (whether patients, providers, or others) is another matter. We recommend that the last bullet point (Numerical representation are uniform…) be removed as it totally unclear.