- Adults (=18 years) with a recent diagnosis of their first episode of idiopathic TTP, who have been treated with plasma exchange with or without other therapies, and who are in remission. Remission is defined as the normalization of platelet counts and lactate dehydrogenase levels with no clinical signs or symptoms of microvascular injury for more than 30 days. The patients will be included in the study within 30 days of remission of their symptoms. Thepatients will all meet the following diagnostic criteria for idiopathic TTP: (1) thrombocytopenia with platelet count <150 x 109 /L, (2) microangiopathic hemolytic anaemia (presence of red blood cell fragmentation by peripheral blood smear), (3) elevation of lactate dehydrogenase (LDH)> 1.25 X of the upper limit of normal, and (4) ADAMTS13 Activity < 10%