What is a key feature of hemophilia A diagnosed by laboratory studies?

Hemophilia A is characterized by a deficiency of factor VIII, which plays a critical role in the intrinsic pathway of coagulation. Laboratory studies typically demonstrate an increased partial thromboplastin time (aPTT) because the intrinsic pathway is primarily affected due to the lack of factor VIII. This prolongation occurs while the prothrombin time (PT) remains normal because the extrinsic pathway, which is assessed by PT, is not affected by the deficiency of factor VIII.

The increase in aPTT is central to the diagnosis of hemophilia A because it reflects impaired coagulation activation that directly corresponds to the missing factor. Thus, the key laboratory feature that helps in diagnosing hemophilia A is the increased aPTT, which guides further management and consideration of factor replacement therapies. Understanding this diagnostic aspect is crucial for recognizing and treating hemophilia effectively.