Which test is primarily used to diagnose Hemophilia A?

The Partial Thromboplastin Time (PTT) is primarily used to diagnose Hemophilia A because this test specifically assesses the intrinsic and common pathways of coagulation, where the deficiencies in factors VIII and IX contribute to the disorder. Hemophilia A is characterized by a deficiency in factor VIII, which plays a critical role in the coagulation cascade.

When PTT is prolonged, it indicates that there is a defect in the intrinsic pathway which is typical in Hemophilia A. Clinicians rely on this test not only to identify the disorder but also to evaluate the severity of bleeding tendencies associated with it. In contrast, the Prothrombin Time (PT) primarily assesses the extrinsic pathway and is not utilized for diagnosing Hemophilia A due to the nature of the factors involved. Bleeding time does not provide specific insights into the coagulopathy associated with hemophilia, and platelet count, while important for assessing overall hemostasis, does not directly relate to factor deficiencies that characterize Hemophilia A. Thus, PTT is the most relevant test in this context.