A mechanism-based way to evaluate commutability of control materials for enzymatic measurements. The example of gamma-glutamyltransferase.

Background: Commutability is an essential requirement for control materials used in external quality assessment schemes. Testing control materials for commutability requires costly and time-consuming experiments. The aim of our work was the validation of a more effective way to evaluate commutability.
Methods: Commutability of two control materials for gamma-glutamyltransferase (GGT) activity between IFCC reference method and four routine methods (Abbott, IL, Roche, Siemens) was tested with the conventional approach and with a new one. The new approach consisted in assessing the effects on the two controls and on two human serum pools, of similar GGT concentrations, of modifications of the measurement conditions (pH, substrate and buffer concentration). The effects of these variations on the two types of materials were compared.
Results: Low GGT activity control was commutable in all method comparisons, the high level control was non-commutable for Abbott and Roche. Both controls showed behavior different from human pools when changing the measurement conditions, but this fact had significant effects only for the conditions of Abbott and Roche methods, confirming the results of the conventional approach.
Conclusions: The proposed approach seems able to identify the reasons for non-commutability and to anticipate the final commutable or non-commutable behavior of the material.
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