Flowchart for the differential diagnosis between hereditary and acquired TTP. Differential diagnosis between the inherited form (USS) and the acquired form of TTP was performed using the following flowchart. By definition, all enrolled patients exhibited an ADAMTS13 activity < 10% at inclusion. During the TTP boot, 23 patients had autoantibodies to ADAMTS13 (acquired TTP), whereas 19 patients had neither anti-ADAMTS13 IgG nor inhibitor (no differential diagnosis between inherited and acquired TTP). Among these 19 patients, 9 recovered a detectable ADAMTS13 activity (> 30%) in clinical remission, allowing the retrospective diagnosis of acquired TTP. In contrast, 10 patients still exhibited an ADAMTS13 activity < 10% in remission with no autoantibody against ADAMTS13, and ADAMTS13 gene sequencing confirmed the diagnosis of USS. Our cohort of 42 obstetrical-onset TTP patients included 10 USS and 32 acquired TTP patients.