[Determination of factor VIII-coagulation antigen in factor VIII concentrates using an inhibitor test]

The method of determination of factor VIII coagulation antigen (VIIICAg) here described is based on the well established factor VIII inhibitor test. Highly concentrated inhibitors to factor VIII in factor VIII substituted hemophiliacs are the most frequent among inhibitors to clotting factors. If such an inhibitor is used in an inhibitor assay its reaction depends on the VIIICAg concentration in the test system. The slope of the reaction curve is proportional to VIIICAg if the inhibitor titer is known. VIIICAg was measured in one cryoprecipitate and 5 different factor VIII concentrates of medium to high purity using this assay. In addition factor VIII related antigen and factor VIII ristocetin cofactor were determined by routine methods. VIIICAg fairly corresponded to factor VIII coagulation activity. Only cryoprecipitate contained significantly more VIIICAg.