ADAMTS13 and thrombotic thrombocytopenic purpura: However, ADAMTS13 antigen was usually only moderately reduced in patients with severe ADAMTS13 activity deficiency and anti-ADAMTS13 antibodies were detected (although at a low titer) in one-fourth of patients with ADAMTS13 ≥ 10%, pointing out that the measurement of ADAMTS13 activity is the only useful test for TMA/TTP differential diagnosis.