Hemostatic effect of platelet von Willebrand factor.

In type III von Willebrand disease (vWD) patients, the bleeding time was only partially corrected or not modified after cryoprecipitate infusion, although the levels and the multimeric structure of plasma von Willebrand factor (vWF) were normal. However, the adhesion of normal platelets on the vessel wall subendothelium in the presence of postinfusion patient plasma improved more significantly than the bleeding time. These results suggest a role of the vWF released from normal platelets which is absent in type III vWD platelets. In 5 patients transfusion of normal platelet concentrates performed 1 h after cryoprecipitate infusion without modification of the bleeding time (> 30 min) normalized this parameter, and platelet adhesion to the subendothelium elicited a marked improvement. These last results confirm the suggestion that platelet vWF plays an important 'in vivo' role in the hemostatic process, particularly in patients suffering from severe vWD.