In mixing studies, what indicates a factor inhibitor is present?

• A. The prolongation completely corrects
• B. The prolongation does not correct or only partially corrects after mixing
• C. Prolongation worsens after mixing
• D. Mixing has no effect on prolongation

Answer: (B)

Mixing studies test whether a prolonged clotting time comes from a missing factor or from an inhibitor present in the patient’s plasma. When you mix the patient plasma 1:1 with normal plasma, a true factor deficiency is resolved because the normal plasma supplies the missing factor, allowing the clotting time to normalize. If a factor inhibitor is present, it acts on the factors in the entire mixture, including those from the normal plasma, so the clotting time does not fully normalize or may only partially improve. This is why not correcting or only partial correction after mixing points to an inhibitor rather than a simple deficiency. Some inhibitors can be time-dependent, showing correction initially and then loss of correction after incubation, which further supports an inhibitor.