Production was attempted for antibodies that neutralize the activity of a
bispecific antibody having F.VIII function-substituting activity, for use in a method for measuring the reactivity of F.VIII in the presence of a
bispecific antibody having F.VIII function-substituting activity. As a result, it was discovered that by using the produced antibodies, F.VIII activity in the
plasma of a hemophilia A patient can be evaluated accurately by performing APTT-based one-stage clotting
assay on a wide range of bispecific antibodies having F.VIII function-substituting activity. It was also discovered that F.VIII inhibitor
titer in the
plasma of a hemophilia A patient carrying F.VIII inhibitor can be evaluated accurately by APTT-based Bethesda
assay.