The inventors produced substances that neutralize the activity of a bispecific antibody having an activity of functionally substituting for FVIII, and undertook the construction of methods for measuring the reactivity of FVIII that can ensure accuracy even in the presence of this bispecific antibody. As a result, the inventors discovered that in APTT-based one-stage clotting assay, FVIII activity in the plasma of a hemophilia A patient can be evaluated accurately, and also that in APTT-based Bethesda assay, FVIII inhibitor titer in the plasma of a hemophilia A patient carrying a FVIII inhibitor can be evaluated accurately.