Procoagulant specificity of factor VIII inhibitor

Nine haemophilia A patients with an inhibitor to factor VIII procoagulant and eight without an inhibitor were studied for the presence of an inhibitor to von Willebrand factor (vWf) in a quantitative ristocetin-induced platelet aggregation system. The mean vWf, factor VIII related antigen (FVIII Ag) and vWf:FVIII Ag ratio were not significantly different in the two groups (P greater than 0.6). The inhibitor plasmas did not reduce the wWf level in normal plasma after a 2 h incubation. The factor VIII inhibitor is highly specific for the procoagulant function of the factor VIII complex.