As hallmark of TTP, generalized hyaline thrombi in the patient's microcirculation is known. These thrombi are composed of platelets and VWF. A severe defect of the VWF cleaving protease (VWF-CP) was found in all known patients with the inherited form of TTP. In contrary, although a severe deficiency of VWF-CP is specific for the acquired form, too, only a fraction of these patients is characterized by a severe deficiency. In most patients with a severe deficiency autoantibodies directed against VWF-CP is detectable in plasma. However, many patients with acquired TTP do not show any severe deficiency. Because treatment differs in inherited and acquired forms and as persistance of autoantibodies during clinical remission is of prognostic value, the determination of the activity of VWF-CP and of antibodies against VWF-CP are important parts in the workup of patients with TTP. In all methods for the determination of the activity of VWF-CP the first step is proteolysis of a specific substrate for the protease. In the second step the activity of the protease is measured by analysis of the residual VWF multimers, by the generation of specific fragments, by using the residual VWF:CB or VWF:RCo as marker of the loss of multimers or with help of specific monoclonal antibodies. In less than 30 min the cone and plate(let) aggregometer helps to distinguish between different forms of thrombotic microangiopathies. While adhesion and aggregation of platelets from a healthy person are clearly enhanced after addition of a small amount of plasma from a TTP patient, both characteristics are weakened by plasma from patients with other forms of thrombotic microangiopathy (dilution effect). Molecular genetics are established methods in the differentiation between inherited and acquired forms of TTP in those cases without autoantibodies against VWF-CP.