In Hemophilia A, mixing with normal plasma will correct the prolonged PTT.

• A. Prolonged PTT remains after mixing with normal plasma
• B. Mixing with normal plasma will correct the PTT
• C. PT shortens after mixing
• D. No effect on PTT

Answer: (B)

The key idea is that mixing studies test whether a prolonged aPTT is due to a missing factor (deficiency) or an inhibitor. In Hemophilia A, there is a deficiency of factor VIII, part of the intrinsic pathway. When you mix the patient’s plasma with normal plasma, the normal plasma supplies the missing factor VIII. This replenishment allows the intrinsic pathway to function again, so the aPTT returns toward normal. If an inhibitor against factor VIII were present, the added factor VIII would be neutralized, and the aPTT would not correct after mixing. The PT, reflecting the extrinsic pathway, would not be corrected by adding intrinsic pathway factors. So the observation that mixing normal plasma corrects the prolonged aPTT fits with a factor deficiency like Hemophilia A.