Platelet von Willebrand factor: an important determinant of the bleeding time in type I von Willebrand's disease.

We studied 17 patients with moderate to mild type I von Willebrand's disease (vWd) and correlated the bleeding time with the plasma von Willebrand factor antigen (vWf Ag), the plasma vWf activity (ristocetin cofactor), the platelet vWf Ag, and the platelet vWf activity. We found an excellent correlation between the bleeding time and the platelet vWf activity and, to a lesser extent, between the bleeding time and the platelet vWf Ag. The length of the bleeding time was inversely proportional to the level of the platelet vWf (P less than .001) or, to a lesser extent, the platelet vWf Ag (P less than .05). The plasma vWf Ag and activity did not correlate significantly with the bleeding time. These studies indicate that the platelet vWf is one of the important bleeding time factors in type I vWd and that the platelet vWf plays an important role in the early steps of hemostasis.