In patients with von Willebrand's disease, which laboratory result indicates increased bleeding risk?

In von Willebrand's disease, the key laboratory finding that indicates an increased bleeding risk is prolonged bleeding time. This is primarily due to the impairment in the function of von Willebrand factor, which plays a crucial role in platelet adhesion to sites of vascular injury. When this factor is deficient or dysfunctional, platelets cannot adhere properly, leading to a longer time for hemostasis and thus a prolonged bleeding time.

In the context of the other laboratory tests: a normal PT (prothrombin time) suggests that the extrinsic pathway of coagulation is functioning properly, and a normal aPTT (activated partial thromboplastin time) indicates that the intrinsic pathway is also intact. A decreased platelet count could indicate thrombocytopenia, but in von Willebrand's disease the platelet count is usually normal; instead, the problem lies in the function of the platelets due to the lack of effective von Willebrand factor.

Thus, the prolonged bleeding time is the indicator of increased bleeding risk in patients with von Willebrand's disease, reflecting the functional issue that leads to excessive bleeding.