Commutability of reference materials in clinical chemistry.

The term commutability applied to reference materials denotes the ability of the material to show interassay changes comparable to those observed in the measurement of the same analyte in human serum. In this study commutability was studied by first establishing the relationship between the results obtained on patient serum samples with two independent methods for a given analyte. Then, the calibration or control material was assayed with the same pair of methods, and its interassay bias was statistically compared with that shown by patient sera. The results obtained in 27 experiments showed that the frequencies of noncommutability in the measurement of some nonenzymic organic components, enzymes, and inorganic ions were, respectively, 25/83, 44/99, and 32/61. The impact of noncommutability of the materials on the results of external quality assessment schemes (EQAS), matrix interferences, and the behavior of particular components are considered. The conclusion drawn is that each material should be tested for commutability before it is used as a reference material.