ADAMTS13 and thrombotic thrombocytopenic purpura: The cause of increased UL-VWF levels in the majority of patients with TTP has been attributed to either congenital defects in a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) or due to the presence of autoantibody inhibitors that compromise the function of ADAMTS13 (4).