The INR: A Perspective

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.

Bleeding and Clotting Disorders

“…Because of individual laboratory reagent variability and the desire to be able to reliably compare the PT from one laboratory to that from another, the PT test is now commonly reported with its INR. 15,16 The INR, introduced by the World Health Organization in 1983, is the ratio of PT that adjusts for the sensitivity of the thromboplastin reagents, such that a normal coagulation profile is reported as an INR of 1.0. 17 This test evaluates the extrinsic coagulation system and measures the presence or absence of clotting Fs I, II, V, VII, and X.…”

From Vitamin K Antagonists to Liver International Normalized Ratio: A Historical Journey and Critical Perspective

“…This variation was demonstrated to be due to the differences in activity of the thromboplastins used. [9][10][11] This led to international standardization efforts that demonstrated a more harmonized outcome when the different thromboplastins were compared with a reference thromboplastin reagent and assigned a sensitivity index, the international sensitivity index (ISI). This was used to derive the international normalized ratio (INR) which rapidly gained approval by the international coagulation community.…”