Inhibitory activity of plasma from patients. Normal plasma was preincubated with equal volumes of heat-inactivated plasma from patients with USS (lanes 2-4), Tc-associated TTP (lanes 5-6), and idiopathic TTP (lane 7). Then, cleavage of GST-VWF73-H was compared with normal plasma without preincubation (lane 1). Plasma from patients with acquired TTP but not with USS inhibited substrate cleavage.