INR (international normalized ratio) is a ratio that helps physicians monitor patients taking anticoagulant medications. The INR is a method of expressing the results of a prothrombin time (PT) blood test. It is based on an international standard that automatically corrects for variations between labs. Thus, using the INR, the PT measurement from one lab can be compared to a PT measurement from any other lab in the world, even if they use different methods to measure PT. The resulting measurement is often referred to as the INR/PT.

The INR/PT was developed specifically for patients who are taking warfarin, an anticoagulant that inhibits the formation of blood clots. Warfarin is prescribed to patients who are at elevated risk of a blood clot causing a heart attack or stroke. The INR/PT test is administered routinely to monitor the blood level of the medication, which must kept within a very narrow range to be effective while lessening risk of uncontrolled bleeding. The INR/PT test may not be effective in patients with liver disease or patients suffering from antiphospholipid syndrome. Similarly, patients who are being given heparin, another anticoagulant given intravenously during surgery or at the initiation of anticoagulation therapy, may not benefit from the test since heparin does not prolong the PT.

Aside from some changes in certain medications that may be ordered by one’s physician, no special preparations are necessary for this test. A healthy person will have an INR of 1.0. However, patients taking anticoagulants to reduce the risk of blood clots may be advised to maintain an INR between 2.0 and 3.5. A higher number indicated greater anti-coagulation. Conditions that may warrant a higher-than-normal INR include atrial fibrillation, artificial heart valves, and thrombophlebitis (e.g., deep vein thrombosis, superficial vein thrombosis). Superficial vein thrombosis is usually not treated with warfarin.