A 46-year-old man has a prolonged aPTT with a normal PT and platelets, and mixing studies return to the control value. Which condition best explains this pattern?

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

Answer: (C)

The key idea is how mixing studies distinguish a factor deficiency from an inhibitor in the intrinsic pathway. A prolonged aPTT with a normal PT and normal platelets points to a problem in the intrinsic pathway rather than platelets or the extrinsic pathway. If mixing the patient’s plasma with normal plasma corrects the aPTT back to normal, this indicates a deficiency of a coagulation factor (the normal plasma supplies the missing factor). Factor XII deficiency is a classic cause of isolated prolongation of the aPTT that corrects with mixing and is not associated with bleeding in vivo. Heparin contamination would prevent correction, and Factor VIII deficiency would typically present with bleeding (and may also correct with mixing, but the bleeding tendency helps distinguish it). Factor XIII deficiency mainly affects clot stability and isn’t the typical cause of a prolonged aPTT. Therefore, the pattern most consistent with this scenario is Factor XII deficiency.