From colleague and friend Heather DeVries, IU Health. Our vote for defining a PTT mixing study correction was "Other." Actually, we use more of a combination of whether the result is in the reference range AND the Rosner index. It is a case-by-case decision. If the clotting times prolong with incubation, we look at the pre and post index. We have seen a couple of acquired inhibitor patients that correct into the reference range and correct in the pre-index, but then have enhanced prolongation and an inhibition post-index. But if everything seems to point to correction (i.e. the clotting time corrects and does not prolong with incubation), we use the pre-index to backup our interpretation of correction. I hope this makes sense!
Thank you, Heather, I appreciate your information and hope others can benefit. Last week Cindy Johns, LabCorp, reminded me of the "saline mixing study." In addition to mixing the patient plasma with reagent normal plasma, you mix a second aliquot with veronal buffered saline. If the immediate control plasma mix result (pre-index) corrects and the saline mix is dramatically prolonged, you may conclude there is either a factor deficiency or a specific factor inhibitor present (having ruled out heparin). Conversely, if the immediate mix result shows no correction and the saline mix is not prolonged, suspect lupus anticoagulant.
I'm eager to learn of others' experience with mixing studies.