Anticoagulation therapy is monitored using the PT-INR test. PT stands for prothrombin time and measures how long it takes blood to clot. The test works by mixing a blood sample with a reagent that triggers clotting and then measuring the time required for a clot to form.
Because different laboratories may use different reagents, PT results can vary. To allow results to be compared across laboratories, the PT value is converted into a standardized measurement known as the INR, or International Normalized Ratio. The INR adjusts for differences in reagents and testing methods, making results more consistent and comparable.
In people with normal clotting function, the INR is close to 1.0. Higher INR values indicate that blood takes longer to clot, while values below 1 suggest a higher tendency toward clotting. Individuals taking anticoagulant medications, such as warfarin, are typically maintained within a therapeutic INR range of approximately 2.0 to 4.0, depending on clinical circumstances. When the INR rises above 5.0, the risk of bleeding increases significantly.
