The INR: a perspective

The use of oral anticoagulants to treat and prevent thromboembolic disease has been a major medical development and has largely been made practicable by the introduction of the prothrombin time as a method for monitoring patients during therapy. Unfortunately, despite many years of application, the administration of oral anticoagulants is still fraught with complications. This has been partly a consequence of the absence of a uniform method of performing and comparing prothrombin times from one laboratory to another. One attempt to solve this problem has been the introduction of the International Sensitivity Index (ISI) and the International Normalized Ratio (INR) as applied to the prothrombin time. The objectives of this article are to (1) explain the need for and development of the INR, (2) discuss shortcomings of the INR and common misinterpretations, and (3) present easy ways to correct for the identified shortcomings.