ADAMTS13 and thrombotic thrombocytopenic purpura: If plasma ADAMTS13 activity is between 10 and 20 U/dL (or 10–20% of normal), clinical judgment is required to interpret this result.18Most adult TTP patients (greater than 95%) are caused by IgG type autoantibodies against ADAMTS13.4, 22A positive inhibitor test (>0.4 U/mL) or an increased level of anti-ADAMTS13 IgG (>15 U/mL) is confirmatory for iTTP.4, 15, 16, 17However, a negative inhibitor or antibody test does not automatically rule out the diagnosis of iTTP.