From Ruth Spates: Hi George, our mixing study procedure requires the patient plasma and the pooled plasma to be incubated separately, and then mixed, and the PT or PTT performed. This is in addition to the 1:1 mix being incubated and retested, and the separately incubated patient plasma and NPP to be retested. I don’t recall the reason why we do this additional mix?? What are your thoughts? Thank you!
Hello, Ruth, and thank you for the question. Operators who use either the Rosner or Chang index to establish their correction/no correction decision limit incubate the patient plasma, normal plasma, and the 1:1 mix in order to apply the formula. The Chang index provides a percentage limit as follows:
% Correction = Patient PTT – 1:1 Mix PTT / Patient PTT – Normal plasma PTT X 100. A typical limit is 75%
Chang SH, Tillema V, Scherr D. A “percent correction” formula for evaluation of mixing studies. Am J Clin Pathol 2002;117:62–73.
Here is the Rosner index, which reports as a ratio:
Rosner index = 1:1 Mix PTT – Normal plasma PTT / Patient PTT X 100. A typical limit is 11
Rosner E, Pauzner R, Lusky A, Modan M, Many A. Detection and quantitative evaluation of lupus circulating anticoagulant activity. Thromb Haemost 1987; 57: 144-147.
Many of us use a percentage difference, typically 10%, or a fixed PTT value such as the upper limit of the PTT reference interval. In this case, we incubate only the normal plasma and the mix and compare the differences.
However, I don’t know of any index or correction/no correction limit that requires testing a 1:1 mix of the incubated patient and normal plasma, perhaps one of our participants uses this approach and can provide an answer.