CALIBRATION VERIFICATION OF THE INTERNATIONAL NORMALIZED RATIO

The International Normalized Ratio (INR) system for reporting the prot hrombin time (PT) is essentially a calibration activity intended to st andardize PT reporting across various reagent/instrument systems, Howe ver, complete standardization of PT reporting through the INR has been difficult to achieve for a variety of reasons, including inaccurate a ssignment of thromboplastin International Sensitivity Indexes (ISIs) a nd specific (local) reagent/instrument effects. Until now, the individ ual laboratory has not been able to easily verify the accuracy of its INR, Using standard lyophilized plasmas with INR values assigned again st IRP RBT/90 rabbit thromboplastin, the authors present a method that allows a laboratory to locally verify its range of accuracy for the I NR. The method is illustrated on a single coagulometer with two thromb oplastin lots of differing sensitivity (Pacific Hemostasis Thromboplas tin-DS and Thromboplastin-D from rabbit sources, with respective Inter national Sensitivity Indexes of 1.20 and 1.97). In this illustration o f the method, the accuracy of Thromboplastin-DS was superior to that o f Thromboplastin-D, Interpretation of the data and cautions regarding the use of standard plasmas for calibration verification are discussed , Using this method, a reportable range of accuracy at a given error t olerance can be established locally for INR measurements within a labo ratory. Laboratories of any size can apply this method to study the ac curacy of their INR reagent/instrument systems, thus performing calibr ation verification, When used in conjunction with assessments of assay precision, this method can help laboratories to select better reagent /instrument systems and thereby produce more accurate and more clinica lly meaningful INR results.