ADAMTS13 and thrombotic thrombocytopenic purpura: These investigations defined a heritable form of TTP with severe (<5%) ADAMTS13 activity deficiency and an acquired form due to the elaboration of antibodies directed at 1 or more ADAMTS13 epitopes.77 However, many thrombotic microangiopathies (TMAs) are not associated either with severe ADAMTS13 activity deficiency or with antibodies that block ADAMTS13 activity.