Poor agreement among prothrombin time international normalized ratio methods: comparison of seven commercial reagents

Prothrombin time (PT) has long been the most popular test for monitoring oral anticoagulation therapy. The International Normalized Ratio (INR) was introduced to overcome the problem of marked variation in PT results among laboratories and the various recommendations for patient care. According to this principle, all reagents should be calibrated to give identical results and the same patient care globally. This is necessary for monitoring of single patients and for application of the results of anticoagulation trials and guidelines to clinical practice.

We took blood samples from 150 patients for whom oral anticoagulation had been prescribed. Plasmas were separated and PTs determined by use of seven commercial reagents and four calibrator sets. The differences in results were assessed by plotting, for each possible pair of methods, the differences in INR values for each sample against the mean INR value (Bland-Altman difference plots).

Mean results differed significantly (P <0.001) for 17 of 21 possible paired comparisons of methods. Only two pairs of methods produced very similar results when assessed for problems of substantial differences in INR values; a significant, systematic increase in the difference with INR; and a significant systematic increase in the variation in difference with increasing INR values.

The agreement among several (and perhaps most) commercial INR methods is poor. The failure of current calibration strategies may severely compromise both the monitoring of individual patients and the application of oral anticoagulation guidelines and trial results to clinical practice.