Von willebrand factor (VWF) is synthesized and released from endothelial cells and megakaryocytes. VWF multimers that are immobilized on the activated endothelial cells and unfolded by high shear stress, undergo proteolysis by specific VWF-cleaving protease, ADAMTS-13. Severe deficiency of ADAMTS-13 is followed by accumulation of ultralarge VWF multimers and development of thrombotic thrombocytopenic purpura (TTP). Several methods for detection of ADAMTS-13 activity are proposed. However, all these assays are lacking the condition of shear stress, in which rapid cleavage of VWF multimeric strings by the ADAMTS-13 occurs. Impact- R method testing the effect of plasma on normal platelet adhesion under shear stress allows rapid screening of acute TTP and discrimination from other thrombotic microangiopathies.