The establishment of International Reference Preparations for thromboplastin together with a recommended methodology for use has permitted the definition of a universal scale for intensity of oral anticoagulation. This scale is called International Normalized Ratio (INR). INRs can be calculated by using calibrated thromboplastin/instrument systems. A calibration model has been developed together with statistical methods to test this model. The imprecision of calculated INRs has been estimated using information from international collaborative studies and proficiency testing programs. Normalized prothrombin times obtained with calibrated thromboplastin/instrument systems have been shown to provide safe patient care in clinical practice. The development of control plasmas with assigned INR equivalents will complete the standardization system. The INR scale will facilitate the process of consensus-making with respect to optimal therapeutic ranges for anticoagulant intensity. Finally, it will improve the continuity of anticoagulant control of travelling patients.