In medical terms, INR stands for International Normalized Ratio. It is a laboratory measurement used to determine the effects of oral anticoagulants such as warfarin on the bodys blood clotting ability. The INR is a standardized number that is calculated based on the results of the prothrombin time (PT) test, which measures how long it takes for a clot to form in a blood sample. The INR is used to monitor individuals who are being treated with the anticoagulant medication warfarin. The INR is preferred over the PT because it is corrected for the strength of the blood clotting tissue and allows for results from different laboratories to be compared. The INR is commonly used as a surrogate for the PT value. The INR number should be between 2 and 3 if you are taking a blood thinner, but it could be different depending on your condition.