International Normalized Ratio: Towards Improved Accuracy

Oral anticoagulants such as warfarin, acenocoumarin, and phenprocoumon induce depletion of vitamin K-dependent coagulation factors, causing prolongation of the prothrombin time (PT) and the International Normalized Ratio (INR). Commercially available thromboplastin reagents used for the determination of the PT vary in their response to coagulation factor depletion, and each is assigned a value of the International Sensitivity Index (ISI) that is used for the calculation of the INR. The INR/ISI system was designed to correct for the PT-ratio differences between thromboplastin reagents and coagulometers. Since the introduction of the INR/ISI system in the middle of the 1980’s, a lot of experience has been accumulated. It is now evident that complete normalization cannot be achieved and that considerable uncertainty is associated with each individual patient’s INR. The intrinsic uncertainty of the INR is caused by the lack of specificity of the PT test. The PT is influenced not only by the coagulation factors VII, X, and II, but also by non-vitamin K-dependent proteins and other substances in the patient’s blood. Thromboplastin reagents vary in their sensitivity and response to coagulation factors which are not depleted to the same extent in each individual patient treated with oral anticoagulants. Thromboplastin reagents are made up with endless variation in tissue factor, phospholipids, salts, buffers, proteins, and preservatives. Many components of the reagent may interact with the patient’s coagulation factors. It is therefore impossible to obtain identical INR values for an individual patient’s sample with all reagents. The only way to reduce the intrinsic INR uncertainty may be achieved by using a single thromboplastin reagent and a single type of coagulometer in all laboratories. This might be possible under a totalitarian regime, but not in the world of free enterprise. Despite the intrinsic limitations of the INR/ISI system, it is of great value to correct the gross differences that remained unobserved before its introduction. Also, the INR is the common scale to express the therapeutic target intensities for various clinical states. In addition to the intrinsic uncertainty of the INR, ISI calibration errors contribute to the overall INR error. Systematic calibration errors can be greater than the imprecision of the calibration procedure itself. The conditions of ISI calibration performed by a thromboplastin manufacturer are not always identical to those of the user, which may be the cause of systematic errors. ISI calibration can be influenced by preanalytical conditions such as citrate anticoagulant concentration and the type of blood collection tubes (1-3). Some studies suggested differences between coagulometers of the same make (4). To correct for systematic calibration errors caused by coagulometers, local system calibration with certified calibrant plasmas has been proposed by various groups (5-7). However, systematic differences caused by preanalytical variation cannot be corrected by certified calibrant plasmas. The only way to eliminate this source of variation is to standardize blood collection systems rigorously. In the present issue of Thrombosis and Haemostasis, Chantarangkul and colleagues, pp. 1621–6, report their experiences with a set of 20 calibrant plasmas used by 92 Italian laboratories for local ISI calibration (8). These laboratories used various thromboplastins and instruments which are representative of the Italian situation. Using this set of calibrant plasmas, the authors could demonstrate a substantial reduction of the interlaboratory variation of the INR. Their results indicate that the systematic differences between the major part of the laboratories could be corrected by means of local calibration. However, there were three laboratories using bovine thromboplastin whose INR drifted from a mean value of 4 (using the stated ISI) to a mean value of 5 (using the calibrant plasma procedure). Did these laboratories err when they used the stated ISI or gave the calibrant plasma procedure an incorrect value? Anyway, the effect of only three laboratories drifting away from the overall mean INR could not reverse the improvement of the CV for all 92 laboratories. Two different international reference thromboplastins had been used to assign certified values to each of the 20 calibrant plasmas in the Italian study: a recombinant human thromboplastin (ECAA) and a rabbit brain thromboplastin (CRM 149S). The INR values calculated with the ECAA reference thromboplastin were significantly lower than those calculated with CRM 149S. One would have expected identical INRs with the two reference thromboplastins, but the mean INR of the 20 calibrant plasmas calculated with CRM 149S was 16% greater than the mean calculated with the ECAA human thromboplastin. This difference was significant and was most probably caused by a different composition of the artificially depleted calibrant plasmas when compared to fresh coumarin plasmas and by the freeze-drying process. A similar difference was observed with the lyophilized test plasmas: local calibration with the rabbit thromboplastin’s certified values resulted in higher INRs than calibration with the human reagent’s values. The interlaboratory variation of the INR seemed to be influenced by the type of lyophilized test plasma (i.e., artificially depleted or coumarin), and the authors suggested additional studies to establish whether the interlaboratory variation measured on fresh patient plasmas can be reduced by the calibrant plasma procedure. Also, the use of frozen calibrant and test plasmas should be explored further, as these may be more similar to fresh plasmas (9, 10). In the authors’ calibration model, the INR of the test plasma was calculated with the slope of the local regression line for the 20 abnorCorrespondence to: Dr. A. M. H. P. van den Besselaar, Dept. of Hematology, Leiden University Medical Center, Rijnsburgweg 10, P.O. Box 9600, NL-2300 RC Leiden, The Netherlands – Tel. : +31 71 526 1894; Fax: +31 71 526 6755 International Normalized Ratio: Towards Improved Accuracy