From Donna Lawler, University of Wisconsin: George, I have been reading with interest the ongoing discussion of PTT reagent selection and enjoyed the corresponding article you cited. I was wondering if you have any similar literature references concerning protime/INR reagents. We use a thromboplastin that has a low ISI (close to 1.0) and good factor sensitivities but we are getting pressure from our lab director to switch to a different reagent because the peer group in the CAP proficiency testing is small and this makes them uncomfortable. What are your thoughts? Thanks, Donna.
P.S. I really enjoy “The Fritsma Factor” website…keep up the good work!
Hi, Donna, and thank you for your question and your kind compliment. There is a recent publication describing thromboplastin standardization,van den Besselaar AM, Chantarangkul V, Tripodi A. Thromboplastin standards. Biologicals 2010;38:430–6. This article describes lot-to-lot reagent validation using a set of reference plasmas, an approach that is mostly unavailable in the US because of FDA restrictions. I suggest that if your reagent performs well, matches well with your instrumentation, and meets your internal validation requirements, there is no reason to change, just because it is not the reagent with the biggest market share. If you have any reason to be concerned about you reagent’s accuracy, you can also check it by comparing to the chromogenic factor X assay. (This is a direct X assay, not the anti-Xa assay we use for heparin.) The method for the assay is provided in McGlasson DL, Romick BG, Rubal BJ. Comparison of a chromogenic factor X assay with international normalized ratio for monitoring oral anticoagulation therapy. Blood Coagul Fibrinolysis. 2008;19:513–7. In fact, there are a few people around who advocate for routine use of the chromogenic factor X in place of the PT, as it is less prone to interference. This probably won’t catch on anytime soon, however, considering how long it takes us to adopt routine anti-Xa for heparin monitoring.
By the way, Herb Crown just posted a comment on PTT mixing studies that provides an excellent summary.