Which of the following tests is primarily abnormal in patients with von Willebrand disease?

In von Willebrand disease, the primary abnormality often observed is in platelet aggregation. This condition is characterized by a deficiency or dysfunction of von Willebrand factor (vWF), which is essential for platelet adhesion to the blood vessel wall during the formation of a platelet plug in response to vascular injury. When vWF is deficient or dysfunctional, platelets are less able to aggregate effectively, leading to problems with clot formation and an increased tendency to bleed.

Although some tests, such as APTT (activated partial thromboplastin time), may also be prolonged due to the role of vWF in stabilizing factor VIII, the most definitive test indicating the abnormality associated with von Willebrand disease is indeed platelet aggregation. Specifically, platelet responses to ristocetin, a reagent used to evaluate vWF activity, are often diminished, providing a clear diagnostic indicator for the condition.

PT (prothrombin time) and fibrinogen levels are generally normal in von Willebrand disease, as the intrinsic mechanism primarily involved in this disorder does not directly affect the common pathway of coagulation where these factors are relevant. Thus, recognizing that platelet aggregation is the primary test affected in this disease highlights its significance in diagnosing and managing von Willebrand disease.