From Dr. Deanna Franke, Carolinas HealthCare System: Thank you so much for all the valuable resource information on your website! My question relates to use of variations of mixing studies--specifically the use of dilutions during mixing studies: 1:1 vs 4:1 and saline mix. In what situations is it appropriate to use these variations?
Hello, Dr. Franke, thank you for your question. The late Dr. Douglas Triplett advocated for the 4:1 mix, which means 4 parts patient plasma to one part pooled normal plasma (NP ). Use the 4:1 mix when there is a clinical indication for an inhibitor, but the customary 1:1 mix result shows correction. This usually applies to suspicion of lupus anticoagulant based upon symptoms, and either the LA has low in vitro avidity or the PTT reagent you are using has intermediate LA sensitivity. An alternative to the 4:1 mix protocol is to switch to an LA-sensitive (low phospholipid) PTT reagent.
Friend and colleague Flo Newlin, who managed the Colorado Coagulation Consultants laboratory, advocates for a 1:1 patient plasma to veronal buffered saline mix. Not many of us use this step today, however it offers the advantage that the PTT of the saline mix becomes markedly prolonged if there is an intrinsic pathway factor deficiency or a specific inhibitor such as anti-factor VIII is present. Conversely, the saline mix corrects when LA is present. Though the saliine mix is not part of today's primary mixing study algorithms, it provides additional validation to your results and conclusion.