What Is the Relationship Between Prothrombin Time and an INR Level Measurement?

The measurement of prothrombin time and INR levels indicate whether a patient is responding appropriately to warfarin, a blood-thinning medication, according to Lab Tests Online. Warfarin requires careful monitoring to prevent the formation of blood clots without causing excessive bleeding. Doctors can adjust a patient's warfarin dosage based on INR levels to achieve a safe prothrombin time.

The prothrombin time, or PT, is a blood test that helps diagnose the cause of unexplained bleeding or inappropriate blood clots, explains Lab Tests Online. For patients on warfarin, laboratories calculate the INR level using the PT results to determine the effectiveness of warfarin therapy. These patients require close monitoring with blood tests and frequent adjustments to their medication dosages to maintain a therapeutic level. Additionally, they must inform their health care providers when they take any other medications or supplements, or consume certain foods, since these may alter PT and INR levels.

An INR level of 1.1 or below is normal in a healthy person, but for patients on warfarin, it should range from 2.0 to 3.0, says Mayo Clinic. An INR that is higher than this range means the blood is clotting too slowly, and a lower INR means the blood is clotting too quickly.