International Normalized Ratio, a system established by the World Health Organization (WHO) and the International Committee on Thrombosis and Hemostasis for reporting the results of blood coagulation (clotting) tests. All results are standardized using the international sensitivity index for the particular thromboplastin reagent and instrument combination utilized to perform the test.

For example, a person taking the anticoagulant ("blood thinner") warfarin (brand name: Coumadin) might optimally maintain a prothrombin time (a "pro time" or PT) of 2 to 3 INR. No matter what laboratory checks the prothrombin time, the result should be the same even if different thromboplastins and instruments are used. This international standardization permits the patient on warfarin to travel and still obtain comparable test results.
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Other sites:

http://www.clotcare.com/clotcare/ptinr.aspx

http://www.heartrhythmfoundation.org/a-fib/INR_FINAL.pdf