The International Normalised Ratio (INR) is a standardised measure of how long it takes for blood to clot, calculated from the prothrombin time (PT). Standardisation across different laboratories is achieved by adjusting for the sensitivity of the thromboplastin reagent used. INR is most commonly used to monitor warfarin (Coumadin) therapy — a widely used anticoagulant drug.
A normal INR is approximately 1.0. For most therapeutic indications, warfarin is dosed to achieve an INR of 2.0–3.0 (or 2.5–3.5 for mechanical heart valves). An INR below the target range means the blood is clotting too quickly — increasing the risk of dangerous blood clots. An INR above the range means the blood is clotting too slowly — increasing the risk of serious bleeding.
FAQs
How often do I need INR testing on warfarin?
INR testing frequency depends on how stable your INR is. Initially, tests are often weekly. Once stable, testing every 4-8 weeks is typical. Any change in dose, diet, medications, or illness may require more frequent testing.
Can I eat green vegetables on warfarin?
Yes, but consistently. Vitamin K in vegetables reduces warfarin's effect. Rather than avoiding vitamin K foods, aim to eat similar amounts each week. Sudden large increases (like eating a large amount of spinach or kale) can lower your INR; sudden decreases can raise it.
What is the difference between warfarin and DOACs?
Warfarin requires regular INR monitoring. Direct oral anticoagulants (DOACs) such as rivaroxaban, apixaban, and dabigatran have predictable effects and do not require routine INR monitoring, which makes them more convenient for most patients. DOACs are now preferred over warfarin for most indications.
What does elevated INR without warfarin mean?
Elevated INR without anticoagulant use indicates impaired production of clotting factors, most commonly from liver disease, vitamin K deficiency, or rarely inherited coagulation factor deficiencies. In acute liver failure, INR is an important prognostic marker.
