we read with interest the letter by van den Besselar about ISI calibrant plasmas (1) and the ensuing exchange of correspondence with Poller et al. (2,3). We agree with the ECAA colleagues that with the FDA-approved ECAA set of artificially depleted plasmas and the PT/INR line inter-laboratory INR deviations are significantly reduced with all types of reagents, but we also suspect that, unless fresh native plasmas from patients on warfarin are used for calibration, this may not translate in better laboratory monitoring of patients on warfarin. © 2012 International Society on Thrombosis and Haemostasis.