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Background Thrombotic thrombocytopenic purpura (TTP) is a rare, life-threatening thrombotic microangiopathy (TMA) requiring urgent treatment. Standardization of its diagnosis and optimal management is challenging. This study aimed to evaluate the role of centralized, rapid testing of ADAMTS13 in patients experiencing acute TMAs requiring plasma-exchange (PEX) and to estimate the incidence of TTP in a large Italian Region. Methods We perfomed a cohort study in the frame of the project “Set-up of a Lombardy network for the study and treatment of patients undergoing apheresis”, including 11 transfusion centers in the Region. Consecutive patients referred from 2014 to 2016 with acute TMAs requiring PEX were enrolled. Centralized ADAMTS13 activity testing was performed at the Milan Hemophilia and Thrombosis Center within 24 h. Results Forty-three TMA patients (44 events) were enrolled, of whom 35 (81%) had severe ADAMTS13 deficiency. Patients with severe ADAMTS13 deficiency were younger, mainly women, with a higher prevalence of autoimmune disorders and a lower prevalence of cancer. Clinical and laboratory characteristics of patients with and without severe ADAMTS13 deficiency largely overlapped, with a lower platelet count being the only baseline marker that significantly differed between the two patient groups (ADAMTS13 activity < 10% vs ≥ 10%: median difference of -27 × 109/l, 95% CI − 37 to − 3). PEX treatment was initiated in all patients, but soon discontinued in cases without severe ADAMTS13 deficiency. In this group, the mortality rate was higher and no episode exacerbations or relapses within 6 months occured. The estimated average annual incidence of acute acquired TTP events was 1.17 [0.78–1.55] per million people. Conclusions Severe ADAMTS13 deficiency distinguished two groups of patients with largely overlapping clinical features but different treatment and disease course. This study provides a feasible model implemented in a large Italian region for the practical clinical approach to TMAs and underlines the importance of urgent ADAMTS13 activity testing for an accurate differential diagnosis and therapeutic approach.

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