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The international normalized ratio (INR) is a standardized number that's figured out in the lab. If you take blood thinners, also called anti-clotting medicines or anticoagulants, it's especially important to check your INR. The INR is figured out using the results of the prothrombin time (PT) test.

Manufacturer of thromboplastin reagent determines ISI. Usual range: 0.9 to 1.7.