The International Normalized Ratio: A Guide to Understanding and Correcting Its Problems

doi:10.1001/archinte.1994.00420030084008

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Vol. 154 No. 3, 14 February 1994

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The International Normalized Ratio

A Guide to Understanding and Correcting Its Problems

Jack Hirsh, MD; Leon Poller, DSc, MD, FRCPath

Arch Intern Med. 1994;154(3):282-288.

Abstract

With the increasing use of the international normalized ratio(INR) to monitor warfarin therapy, a number of problems withprothrombin time (PT) testing have been identified that haveled some laboratory physicians to question the reliability ofthe INR.¹ This is ironic, because it was the introduction ofthe INR system that brought to light some of the long-standingproblems with the technique of PT monitoring. However, theseproblems are not insurmountable if a compromise can be reachedbetween the expectations of laboratory physicians and of clinicians.Thus, the laboratory physician seeks a perfect assay system,which in the case of the INR is unattainable at present, becauseof differences in PT reagents and methods. In contrast, theclinician is satisfied with a system of monitoring that providessafe and effective warfarin dosing. This goal can be achievedprovided that certain details of PT testing are observed. Inthis communication, which is directed to practicing clinicians,the potential problems with the INR system are discussed, theirclinical relevance is critically reviewed, and solutions areoffered.

Author Affiliations

From the Hamilton Civic Hospitals Research Centre and the Department of Medicine, McMaster University, Hamilton, Ontario (Dr Hirsh); and the Department of Pathological Sciences, Medical School, University of Manchester, England (Dr Poller).

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