Determining the adequate number of internal quality control levels: the example of coagulation factor VIII assay

Taking the specific case of coagulation factor VIII assay, we determined the characteristics of an internal quality control panel assuring control of the assay method for all of the critical factor VIII concentrations. The precision of the assay method was determined on six control materials C1-C6, with expected factor VIII levels of 1, 5, 30, 50, 80 and 150 U/dl, respectively. Given that, when two control levels correlate statistically, the information provided by one of them is redundant, we used correlation and principal components analysis to define a priori adequate and inadequate control panels. For each of these panels, we calculated the number of runs required, using Hotelling's method, to detect a shift expressed on C1 and impacted on C2, C3, C4, C5 and C6 in relation to the correlation phenomena among the six levels. The C1/C6 panel proved to be as informative in this regard as the complete panel for a 1 U/dl shift simulated on C1 and impacted on other levels too. These correlation phenomena allow the biologist to implement fewer control levels than there are critical concentrations needing to be explored in the internal quality control plan.