Introduction Thrombotic thrombocytopenic purpura (TTP) is a rare life-threatening thrombotic microangiopathy (TMA) characterized by the severe deficiency of ADAMTS13 activity ( Methods A comparison method was performed to compare HemosIL Acustar® with an in-house FRETS-VWF73 assay and two commercial ELISA assays: the TECHNOZYM® ADAMTS13 Activity (Technoclone GmbH, Vienna, Austria) and the DG-EIA ADAMTS-13 Activity (Diagnostic Grifols, SA, Barcelona, Spain). A set of 241 frozen plasma samples with known ADAMST13 levels was used. Agreement between methods was assessed with focus on two cut-off ADAMTS13 activity values: Results HemosIL AcuStar® showed high agreement with the other methods in correctly classify patients with ADAMTS13 values below 10% (Kappa = 0.89) and even below 5% (Kappa = 0.94) with no false negatives and few false positives (5.40%; 95% CI: 2.20 to 8.60%). However, it also tended to underestimate ADAMTS13 levels, especially for the high assay range values (>40%) (absolute mean bias of 8.40% (95% CI: 6.53 to 10.42%)) when compared to other assays. Conclusions HemosIL AcuStar® is highly sensitive to detect ADAMTS13 values below 10% and 5%. A large prospective validation study is needed to corroborate its utility in clinical practice.