Which condition is classically associated with prolonged PTT and normal PT?

Which condition is classically associated with prolonged PTT and normal PT?

• A. Haemophilia
• B. Von Willebrand disease
• C. Thrombocytopenia
• D. Aspirin

Answer: (A)

The main idea is how coagulation tests map to the clotting pathways. PTT (aPTT) assesses the intrinsic pathway, while PT assesses the extrinsic pathway. In haemophilia, there is a deficiency of factors VIII or IX in the intrinsic pathway, so the intrinsic pathway is impaired and the aPTT is prolonged. But the extrinsic pathway remains intact, so the PT stays normal. That classic pattern—prolonged aPTT with a normal PT—points to a defect in the intrinsic pathway, with haemophilia A or B as the prototypical cause. Von Willebrand disease can sometimes prolong the aPTT because vWF stabilizes factor VIII, but the association isn’t as consistent or classic as haemophilia. Thrombocytopenia affects platelet counts and function rather than the clotting factors measured by PT/PTT, so those tests aren’t the hallmark. Aspirin affects platelet function and bleeding time rather than the coagulation factors, so PT and aPTT are typically unchanged.

The main idea is how coagulation tests map to the clotting pathways. PTT (aPTT) assesses the intrinsic pathway, while PT assesses the extrinsic pathway. In haemophilia, there is a deficiency of factors VIII or IX in the intrinsic pathway, so the intrinsic pathway is impaired and the aPTT is prolonged. But the extrinsic pathway remains intact, so the PT stays normal. That classic pattern—prolonged aPTT with a normal PT—points to a defect in the intrinsic pathway, with haemophilia A or B as the prototypical cause.

Von Willebrand disease can sometimes prolong the aPTT because vWF stabilizes factor VIII, but the association isn’t as consistent or classic as haemophilia. Thrombocytopenia affects platelet counts and function rather than the clotting factors measured by PT/PTT, so those tests aren’t the hallmark. Aspirin affects platelet function and bleeding time rather than the coagulation factors, so PT and aPTT are typically unchanged.