The aim of this study was to analyze the inhibitory anti-ADAMTS13 antibody assay to understand why currently published Bethesda assay protocols in TTP require a 2-hour incubation period, similar to factor VIII antibodies,9, 10but not normally undertaken for other inhibitory antibodies when reduced coagulation factor levels are detected. The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.