What does an increased aPTT indicate in a patient with suspected von Willebrand's disease?

An increased activated partial thromboplastin time (aPTT) in a patient with suspected von Willebrand's disease is indicative of a coagulation factor deficiency. In von Willebrand's disease, there is a deficiency or dysfunction of von Willebrand factor (vWF), which plays a crucial role in platelet adhesion and is also a carrier for factor VIII, an important coagulation factor. When vWF is deficient, there can be an inadequate level of factor VIII in circulation, leading to a prolonged aPTT because the intrinsic pathway of the coagulation cascade is affected.

In this context, the elevated aPTT reflects the impaired function in the clotting process due to either the absence of vWF or a functional defect in its action, which subsequently leads to a deficiency in factor VIII. This is a key marker in the evaluation of bleeding disorders and supports the diagnosis of von Willebrand's disease.