Abstract
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.
Keywords:
International Normalized Ratio (INR) - International Sensitivity Index (ISI) - prothrombin
time ratio (PTR) - prothrombin time (PT) - thrombosis
