What a mix is actually testing

PT and aPTT are clot-based screening tests: reagent activates the plasma and the analyzer times how long fibrin clot formation takes. A prolonged time can come from too little of a needed clotting factor, or from a substance in the plasma that interferes with the reaction regardless of how much factor is present. A mixing study distinguishes these two mechanisms by diluting the patient's plasma with plasma known to have adequate factor activity and asking whether the clotting time normalizes.

Combine equal volumes of patient plasma and NPP. If the patient's prolongation is caused by a missing factor, the NPP supplies enough of that factor that the mixture clots close to normal; this is a corrected mix and it supports a deficiency pattern. If the prolongation is caused by an inhibitor, the inhibitor is still present in the diluted mixture and continues to interfere, so the mixture stays prolonged; this is a noncorrecting mix and it supports an inhibitory pattern. A corrected mix supports factor deficiency but does not say which factor is low, and a noncorrecting mix supports an inhibitor but does not identify which one; it could be LA, a specific factor inhibitor, heparin, or a DOAC.

NPP itself has to be trustworthy for this logic to hold. Guidance describes NPP with factor activity greater than 80% and low residual platelets, most often pooled from a large donor group, commonly cited around 40 donors for a well-characterized commercial or in-house pool; CAP separately expects a pool of at least 20 apparently healthy donors as part of using an appropriate pooled plasma. A single normal patient's plasma is not an acceptable substitute for NPP, because that one plasma testing normal does not assure it carries adequate activity of every factor the mix might need to correct.

A mixing study answers one question, replacement versus inhibition, and it answers that question only as well as the NPP behind it, so NPP quality review is part of every mix, not an afterthought.