The significance of the International Normalized Ratio (INR) for oral anticoagulant therapy.

For the laboratory control of oral anticoagulant treatment, the prothrombin time (PT) is used universally as the primary measurement. Important variability of the PT ratio (patient PT: mean health persons' PT) caused by different brands and batches of thromboplastin and instruments exists not only in North America, but throughout the world. To achieve uniformity of reporting, it is recommended that the PT ratio be transformed to the International Normalized Ratio (INR). This is possible if the thromboplastin manufacturer provides calibration data for each batch of thromboplastin. The INR scale facilitates the achievement of consensus on optimal therapeutic ranges for various clinical conditions. The INR will further improve the stability of anticoagulant treatment of travelling patients.