If a mixing study corrects an isolated prolonged aPTT, what is the most likely cause?

• A. Factor deficiency.
• B. Inhibitor presence such as lupus anticoagulant.
• C. Vitamin K deficiency.
• D. Heparin contamination.

Answer: (A)

When a mixing study corrects an isolated prolonged aPTT, it means the problem is a lack of one or more intrinsic pathway clotting factors rather than an inhibitor in the plasma. The normal pooled plasma provides the missing factor(s); once those factors are supplied, the coagulation cascade can proceed and the time to clot becomes normal. In other words, there’s no active inhibitor interfering with the factors in the mixture, so the prothrombin time remains corrected.

This is opposed to an inhibitor scenario, such as lupus anticoagulant or a factor-specific inhibitor, where the added normal plasma brings in the missing factors but an inhibitor in the patient plasma (or the inhibitor acting in the mixture) continues to disrupt the cascade, so the aPTT does not normalize or only partially corrects after incubation.

Vitamin K deficiency tends to affect multiple pathways and often prolongs both PT and aPTT rather than producing an isolated aPTT prolongation. Heparin contamination can prolong the aPTT as well, but mixing studies don’t reliably rule it in or out on their own; specialized tests or neutralization steps are used to address heparin. The clear, isolated correction with mixing most strongly points to a factor deficiency.