Assessing analytical accuracy through proficiency testing: have effects of matrix been overstated?

To the Editor: We read with great interest the report of Stepman et al. (1) identifying analytical errors in commercial analytical systems by using an external quality assessment (EQA) scheme with single-donor specimens. The use of such materials provides an assessment of method accuracy that is generally regarded as the gold standard in such comparisons (2). Unfortunately, only a limited number of laboratories can typically participate in such assessments without resorting to pooled or processed proficiency materials. Analytical ranges are also usually limited using single-donor specimens unless these materials are supplemented with additions that potentially obviate the benefits of using such specimens. Although intermethod bias cannot be definitively assessed with materials that have not been shown to be commutable for the examined analytes and analytical techniques, it is incorrect to assume a priori that pooled or spiked specimens, by their nature, are noncommutable and that method biases seen with such specimens are largely due to matrix effects. The data provided by Stepman et al. (1) allow for comparison of bias estimated with more …