Thrombotic microangiopathy can be caused by several conditions which are difficult to diagnose from the clinical presentation alone. Deficient enzyme activity of a newly-discovered enzyme, ADAMTS-13, can lead to thrombotic thrombocytopenic purpura (TTP). Lack of ADAMTS-13 activity causes increased concentrations of high molecular weight von Willebrand factor forms and increased platelet aggregation. Measurement of ADAMTS-13 activity is useful for the diagnosis of TTP and may also be relevant as a prognostic test for recurrent TTP.