INR stands for international normalized ratio. The INR provides some information about a person’s blood’s tendency to clot (which is often described as how “thin” or “thick” their blood is).
The INR comes from the conversion of another value, the prothrombin time (PT). Prothrombin time is the time it takes for blood to clot in a test tube. A high or low PT can suggest bleeding or clotting tendencies. Most often, the PT is checked to monitor a person’s response to blood thinners such as warfarin (Coumadin).
The problem with prothrombin time testing is that several manufacturers produce and sell slightly different PT tests to laboratories. The same person’s blood might have different results if tested on two different companies’ PT tests.
To solve this problem, the international normalized ratio (INR) was created. By entering a PT result into a math formula, it yields the INR. The international normalized ratio means one lab’s test results can be compared to another’s with high accuracy.
A normal INR is approximately 1.0. People taking the blood thinner warfarin typically have a target INR of 2.0 to 3.0. People with an INR higher than the normal range who aren’t taking warfarin may have a medical condition that needs further evaluation. A low INR is rarely significant.
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