Annual incidence of acute TTP episodes in 87 confirmed hTTP patients based on prospective follow-up until their last visit up to the end of 2019. (A) Patients grouped according to their annual incidence of acute TTP episodes. Although 44 patients with hTTP (50.6%; 27 male subjects, 17 female subjects) experienced no acute episode during follow-up, an annual incidence of >0 to 0.5 was documented in 19 (21.8%; 8 female subjects), of >0.5 to 1 in 16 (18.4%; 11 female subjects), and of >1 in 8 (9.2%; 5 female subjects) patients with hTTP. Annual incidence of acute TTP episodes in individual patients according to age at enrollment: patients were receiving plasma prophylaxis (n = 54) (B) or had no plasma prophylaxis (n = 29) (C) at the last follow-up visit. For 4 patients, information on plasma prophylaxis was missing. Annual incidence of acute TTP episodes in relation to ADAMTS13 activity in 69 patients with hTTP having undergone ADAMTS13 activity measurements by using the modified FRETS-VWF73 assay.^25,26  To assess baseline (endogenous) ADAMTS13 activity, blood samples were withdrawn at least 14 days after the last administration of plasma-containing blood products. Patients were receiving plasma prophylaxis (D) or had no plasma prophylaxis (E) at the last follow-up. Annual incidence of acute TTP episodes in relation to interval of regular plasma prophylaxis (F) and to the mean dose of plasma administered per kilogram body weight and year in 78 patients with hTTP with complete data sets for this analysis (G). The 2 patients regularly treated with a plasma-derived factor VIII concentrate are shown separately.