Addressing property and unit discordance in laboratory interoperability: the role of NPU as a governance layer

Unified Code for Units of Measure (UCUM) encodes syntax without prescribing which unit should be used. The Nomenclature for Properties and Units (NPU) terminology, developed by IFCC and IUPAC, specifies both the kind-of-quantity (e.g., mass concentration or substance concentration) and the recommended unit for each code, and can serve as a governance layer, yet no study has evaluated how NPU-recommended units align with actual laboratory practice.

30 clinical chemistry measurands in blood were selected from the overlap between a Korean multi-institutional unit-usage survey and a consensus study on commonly tested analytes mapped to LOINC. For each measurand, commonly used units in Korean laboratories, RCPA-recommended units, and NPU-recommended units were compared, along with LOINC example units.

Of the 30 measurands, 6 (20 %) were fully concordant across Korean practice, Australian practice, and NPU. For 15 (50 %), a kind-of-quantity discordance was observed: Korean laboratories reported mass concentration (mg/dL), whereas Australian practice and NPU used substance concentration (mmol/L). For 6 (20 %), Korean laboratories used IU/L while Australian practice and NPU used U/L. Korean and Australian LOINC code selections overlapped for only 15 (50 %), reflecting different national choices of kind-of-quantity; Korean practice uniformly matched LOINC example units, whereas Australian practice deviated in three instances. NPU did not provide mass concentration codes for the 15 discordant measurands that are reported in mass concentration units in Korea.

The predominant discordance reflects a lack of international agreement on kind-of-quantity selection. A stepwise transition strategy - from unit symbol correction to kind-of-quantity transition - can be informed by the degree of discordance.