Effect of plasma from a congenital TTP patient on VWF-mediated and activation-independent platelet thrombus formation under flow. Normal washed blood cells were resuspended either in the plasma of a healthy individual (control plasma) or that of a patient with TTP obtained from therapeutic plasmapheresis (patient plasma). The bar graph shows the average count, length, and area (mean ± SEM) of all the platelet aggregates present in 5 separate fields of view measured after a 10-minute perfusion. Only the difference in aggregate count was significant (P < .001).