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People taking an anticoagulant medication often have a laboratory test done to make sure their international normalized ratio (INR) is within normal limits. This ratio is a measurement of how long it takes the blood to clot. A high ratio may indicate a risk for uncontrollable bleeding. If the ratio is too low, a person may have a high risk for developing a blood clot.

What Is the INR Test?

The international normalized ratio (INR) is a laboratory test used to monitor people who are taking oral blood-thinning medicines, like warfarin (Coumadin®, Jantoven®). The INR was introduced in the early 1980s by the World Health Organization (WHO) to help ensure the safety of using blood-thinning medications, also called anticoagulants.

When Is the INR Test Used?

In most cases, INR testing is used to monitor people who are taking warfarin. It is generally of little use for monitoring the effects of other anticoagulants. Occasionally, a healthcare provider will check a person's INR if there is some suspicion of a decreased ability of the blood to clot due to a disease or condition, such as liver failure.

How Is the Test Performed?

A healthcare provider usually performs the INR test in a hospital or clinic. A small blood sample is taken either from a finger prick or a standard blood draw. The international normalized ratio is established by measuring the time it takes the blood to clot and then comparing it to an average.
In some cases, your test results will come back quickly; in other cases, it may take longer, depending on whether the blood sample is sent to a laboratory or if testing can be done in the clinic.
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Last reviewed by: Arthur Schoenstadt, MD
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