One-Stage Factor VIII Assays

Coagulation factor assays using one-stage methodology are widely used to measure factor levels for the purpose of detecting a deficiency, or to monitor replacement therapy. In this chapter, we focus on a method to measure Factor VIII coagulant activity (FVIII:C) by the one-stage assay (FVIII:C-1), with extra information provided to also allow this method to apply to Factor IX (FIX), Factor XI (FXI), and Factor XII (FXII). From the perspective of laboratory testing, these factors are components of the "intrinsic" coagulation pathway and are all measured in test systems based on the correction of the Activated Partial Thromboplastin Time (APTT). Factor activity is assessed by measuring the ability of an unknown sample to correct the prolonged APTT of factor-deficient plasma (deficient in the factor of interest), relative to the effect of a known calibrator. These assays are used for the diagnosis of the many causes of reduced factor levels, including those causing a prolonged APTT. It is important for laboratory staff to understand the impact of method variations, limitations, and result interpretation and these aspects are also discussed.