If a prolonged aPTT does not correct with mixing, which explanation is most likely?

If a prolonged aPTT does not correct with mixing, which explanation is most likely?

• A. Heparin contamination
• B. Factor deficiency
• C. Factor inhibitor
• D. Vitamin K deficiency

Answer: (C)

Mixing studies test whether a prolonged aPTT is due to a factor deficiency or to an inhibitor. If you mix patient plasma with normal plasma and the aPTT normalizes, the issue was likely a missing or low clotting factor. If the prolonged result persists after mixing, it suggests there’s an inhibitor in the patient’s plasma that blocks coagulation factors even after dilution. A factor inhibitor works by actively inhibiting the activity of clotting factors, so adding normal plasma doesn’t restore function and the aPTT remains prolonged. Vitamin K deficiency or a true factor deficiency would usually correct with normal plasma because the missing or deficient components are supplemented. Heparin contamination can prolong the aPTT as well, but the classic teaching for a non-correcting mixing study is the presence of a factor inhibitor, which explains the persistent delay.

Mixing studies test whether a prolonged aPTT is due to a factor deficiency or to an inhibitor. If you mix patient plasma with normal plasma and the aPTT normalizes, the issue was likely a missing or low clotting factor. If the prolonged result persists after mixing, it suggests there’s an inhibitor in the patient’s plasma that blocks coagulation factors even after dilution.

A factor inhibitor works by actively inhibiting the activity of clotting factors, so adding normal plasma doesn’t restore function and the aPTT remains prolonged. Vitamin K deficiency or a true factor deficiency would usually correct with normal plasma because the missing or deficient components are supplemented. Heparin contamination can prolong the aPTT as well, but the classic teaching for a non-correcting mixing study is the presence of a factor inhibitor, which explains the persistent delay.