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. Abbreviated INR. Under the INR system, all results are standardized. For example, a person taking the anticoagulant warfarin (brand name: Coumadin) would regularly have blood tested to measure the INR. The INR permits patients on anticoagulants to travel and obtain comparable test results wherever they are.