Coagulation factor VIII was assayed around the critical concentration of 80 U/dl, which is optimal for postoperative haemostasis in haemophiliac patients, in order to assess the use of Bayesian logic in interpreting internal quality control results during a change of reagent or control batch. A mathematical model based on Bayesian inference, requiring no preliminary control-plan phase, was compared with a classical approach, which necessarily involves performing a preliminary phase. Tsiamyrtzis and Hawkins' Bayesian model proved applicable to rapid statistical control of factor VIII assay, detecting shift at least as efficiently as classical approaches, which depend on running the kind of costly and controversial preliminary control phase recommended by Shewhart.