VWF and thrombotic thrombocytopenic purpura: Severely deficient ADAMTS13 to less than a 10% level or activity results in the rare life-threatening disease of thrombotic thrombocytopenic purpura (TTP), in which an abnormal accumulation of HMW-VWF multimers results in greatly increased platelet binding and formation of thrombi that promote microvascular occlusion [7].