Flow chamber analysis of platelet adhesion and thrombus formation under conditions of arterial shear. Mepacrine-labeled platelets from the patient and a healthy control were placed in parallel-plate flow chambers and perfused at arterial wall shear rates of 1500 s^–1 over glass slides coated with VWF or collagen. While patient platelets behave similarly to those from a healthy control when flowed over immobilized VWF (bottom panels), patient platelets adhere to and form thrombi poorly on immobilized collagen (top panels).