INR most commonly stands for international normalized ratio. It is a way of standardizing the results of the prothrombin time test, regardless of the testing method, according to Web MD.
Prothrombin time is a blood test that measures how long blood takes to clot. The prothrombin time result is often reported as INR so that results from different laboratories using different testing methods may be comparable.
A normal person would usually have an INR of between 0.8 and 1.1, according to the Medline Plus Medical Encyclopedia. A person who is taking a blood thinning medication called "warfarin" to prevent blood clots would need to maintain an INR of between 2 and 3.