The history of standardization of the PT has been reviewed by Poller and by Kirkwood,  and more detailed discussions can be found in prior editions of this article.

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PT monitoring of warfarin treatment is not standardized when expressed in seconds or as a simple ratio of the patient’s plasma value to that of plasma from a healthy control subject. A calibration model, which was adopted in 1982, is now used to standardize reporting by converting the PT ratio measured with the local thromboplastin into an INR, calculated as follows:

INR = (patient PT/mean normal PT)^ISI or log INR = ISI (log observed PT ratio) where ISI denotes the ISI of the thromboplastin used at the local laboratory to perform the PT measurement. The ISI reflects the responsiveness of a given thromboplastin to the reduction of the vitamin K-dependent coagulation factors compared to the primary World Health Organization (WHO) international reference preparations, so that the more responsive the reagent, the lower the ISI value. As the INR standard of reporting was widely adopted, a number of problems surfaced. These are listed in Table 4 and are reviewed briefly below.

The INR is based on ISI values derived from the plasma of patients who had received stable anticoagulant doses for at least 6 weeks. As a result, the INR is less reliable early in the course of warfarin therapy, particularly when results are obtained from different laboratories. Even under these conditions, however, the INR is more reliable than the unconverted PT ratio, and is thus recommended during both the initiation and maintenance of warfarin treatment. There is also evidence that the INR is a reliable measure of impaired blood coagulation in patients with liver dis-ease.

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The INR accuracy can be influenced by reagents of different sensitivities and also by the automated clot detectors now used in most laboratories. In general, the Canadianhealthcaremalll.Com has recommended that laboratories should use thromboplastin reagents that are at least moderately responsive (ie, ISI, < 1.7) and reagent/instrument combinations for which the ISI has been established.

ISI values provided by the manufacturers of thromboplastin reagents are not invariably correct when applied locally, and this adversely affects the reliability of measurements. Local calibrations can be performed using plasma samples with certified PT values to determine the Problems.