The most likely cause of the prolonged aPTT in this patient with no bleeding history is which of the following?

• A. Heparin contamination
• B. Factor VIII deficiency
• C. Factor XII deficiency
• D. Factor XIII deficiency

Answer: (C)

In vitro testing of the intrinsic pathway uses contact activation to start clotting. When a prolonged aPTT appears in a patient who has no history of bleeding, a defect in a contact-factor that affects the lab assay but not actual hemostasis is a classic clue. Factor XII is one of these contact factors. If Factor XII is deficient, the intrinsic pathway takes longer to activate in the test, so the aPTT is prolonged. But in living clotting, Factor XII isn’t essential for forming a stable clot, so overall bleeding tendency is not increased.

The other options don’t fit as neatly. Heparin contamination would prolong the aPTT because it acts as an anticoagulant in vivo and in vitro, but the question emphasizes the absence of bleeding history rather than an evident pharmacologic or procedural cause. Factor VIII deficiency (hemophilia A) typically presents with spontaneous or easy bleeding and joints/mucosal bleeds, which contradicts the scenario. Factor XIII deficiency affects cross-linking of fibrin and leads to issues with clot stability and can cause delayed bleeding, but it does not reliably prolong the aPTT in standard testing because the initial clot formation can still occur.