Background: Medical laboratories are required to participate in interlaboratory comparisons of the analyses they perform. The materials used in these comparisons need to be of sufficient quality so that the comparison provides a picture of the performances. One of the main characteristics of the testing material is commutability, which is the ability of a material to yield the same numerical relationships between results of measurements as those relationships obtained when the same procedures are applied to patient samples. The aim of this study was to assess the commutability of 3 different matrices for the preparation of proficiency testing material (PTM) for the analysis of carbamazepine and valproic acid. Methods: Patient samples and PTM containing various concentrations of carbamazepine and valproic acid were collected, prepared, and shipped to different laboratories for analysis. Reported results for patient samples from each laboratory were plotted against results for patient samples of each of the other laboratories, and the corresponding regression line was calculated. The distance of results from PTM to the regression line is a measure for commutability. The distance is expressed as a multiple of the SDwl (average within-laboratory SD as calculated from external quality assessment scheme results) and referred to as relative residual. A commutability decision limit of 2 SDwl was set. Results: For carbamazepine and valproic acid, a total of 78 and 105 laboratory couples respectively could be formed. The number of relative residuals for liquid human serum outside the commutability decision limit was 1, 4, and 0 for low, medium, and high concentrations of carbamazepine, respectively and 3, 1, and 0 for low, medium, and high concentrations of valproic acid, respectively. In both liquid and lyophilized bovine sera, the number of relative residuals outside the commutability decision limit was between 2 and 15 and between 6 and 21 for carbamazepine and valproic acid, respectively. Conclusions: Although not all ...