What would you expect in mixing studies for a patient with a factor VIII inhibitor?

• A. Shortened aPTT; normal factor VIII; presence of inhibitor.
• B. Prolonged aPTT that does not correct with 1:1 mixing; decreased factor VIII activity; presence of inhibitor.
• C. Prolonged aPTT that corrects with 1:1 mixing; increased factor VIII activity; absence of inhibitor.
• D. Normal aPTT; normal factor VIII; presence of inhibitor.

Answer: (C)

Mixing studies help distinguish a true factor deficiency from a circulating inhibitor. If an inhibitor against factor VIII is present, adding normal plasma (a 1:1 mix) does not neutralize the problem; the inhibitor will inactivate factor VIII from the normal plasma as well, so the prolonged aPTT persists. The factor VIII activity measured would be decreased because the inhibitor is neutralizing the added factor VIII, and an inhibitor would be detectable on further testing.

Therefore, in a patient with a factor VIII inhibitor you’d expect a prolonged aPTT that does not correct with 1:1 mixing, decreased factor VIII activity, and evidence of an inhibitor. The alternative scenario—the aPTT correcting with mixing and normal or increased factor VIII activity without an inhibitor—aligns with a simple factor deficiency rather than an inhibitor.