TTP is a rare but potentially fatal disease unless plasmapheresis is initiated promptly. Although most hospitals will only treat such patients sporadically, unless the possibility of TTP is explored, the diagnosis may be missed. In patients with unexplained severe thrombocytopenia (usually less than 10,000 platelets/microL) and hemolytic anemia with schistocytes, TTP must be ruled out. The presence of other clinical and laboratory features as described in Table 1 may aid in the differential diagnosis. Often, however, TTP remains a possibility and the patient should be transferred to a facility where plasmapheresis is available. Further, a sample of citrated plasma should be collected prior to any plasma transfusion, and sent frozen to a reference laboratory for determinations of the vWF-CP activity and the presence of an inhibitor to the enzyme. The clinical laboratory should be aware of this test to help ensure that the sample is correctly collected and shipped. Although the patient should be treated as if having TTP until the result of the tests is available, the assays may help determine the specific diagnosis, as well as the potential need for further therapy as discussed above. We anticipate that continued accumulation of vWF-CP data with clinical correlation will further define the role of this enzyme in the diagnosis and management of patients with TMAs.