Skip to contentSkip to navigation

Prothrombin Time and International Normalized Ratio


PT INR (prothrombin time) is mainly used to monitor patients under chronic treatment with warfarin (Coumadin®), an agent that prevents the formation of blood clots in at-risk individuals (atrial fibrillation, artificial heart valves, deep vein thrombosis [phlebitis], pulmonary embolism, coagulation abnormalities such as antiphospholipid syndrome, occasionally after a myocardial infarction). PT INR is used to adjust the dose of warfarin to achieve a balance between clotting risk and excessive bleeding.

A PT INR greater than 4.5 is problematic and should be reported to the attending physician without delay. Other results should be interpreted based on treatment targets, which differ from one clinical condition to another. Diseases and some medications may alter PT INR. Alcohol and certain vitamin K-rich foods such as beef or pork liver, green tea, broccoli, chickpeas, kale, turnip fans and soybeans can change the result. Difficulties at the time of collection may also interfere.

Term of the Week

Predictive medicine

Medicine that links medical knowledge with data to predict a patient’s potential health problems. Examples include artificial intelligence and genetics.