Guidelines on preparation, certification, and use of certified

on preparation, certification, and use of certified plasmas for ISI calibration and INR determination – reply to a rebuttal. This issue, pp 2371–2. In the November issue (2004) of the Journal of Thrombosis and Haemostasis, Van den Besselaar et al. [1] present guidelines for the use of certified plasmas for ISI calibration and direct INR determination. The guidelines reflect the official standpoint of the Subcommittee on Control of Anticoagulation of the Scientific and Standardization Committee of the ISTH and give clear instructions for both manufacturers and users of certified plasma samples. The authors describe two main procedures of using certified plasmas and the guidelines are backed-up from experiences of various well-defined multi-centre studies or occasionally full-scale field studies. However, it was with disappointment that we learned that the authors only discussed results concerning plain thromboplastin rea-gents (Quick-PT type of assay) and left out experiences wit