Modification of the Bethesda assay for factor VIII or IX inhibitors to improve efficiency

A factor IX inhibitor was assayed by using a modification of the Bethesda assay for factor VIII inhibitors. The incubation time was shortened to 15 min. A screening method using the activated partial thromboplastin time was used after sample incubation to determine the correct dilution of the patient's plasma to assay for residual factor IX activity prior to determining the inhibitor titer, thereby significantly reducing the number of factor assays needed. This screening method was also shown to be applicable to assaying a factor VIII inhibitor.