What Is the Connection between Warfarin and INR?

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  • Written By: wiseGEEK Writer
  • Edited By: Jessica Seminara
  • Last Modified Date: 20 October 2018
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The medication called warfarin and INR, or the International Normalized Ratio, are connected because people who take the drug need to have regular INR readings. Warfarin is an anti-clotting drug that has what is called a narrow therapeutic index. This means it can be easy to exceed the recommended amounts, especially because the drug is extremely reactive and carries the risk of excessive bleeding. Information from the INR tells doctors whether patients are taking a safe and appropriate amount of the drug.

Some readers may conclude that warfarin and INR readings aren’t always connected because they use the drug and have Prothrombin Time (PT) tests. The INR is simply an application of a mathematical formula to a PT. It helps normalize results internationally, as these may be slightly influenced by test materials and the manufacturer. Either test name may be appropriate, and the two terms are often used interchangeably. The only appreciable difference between an INR and PT is whether the results are normalized; the blood test itself, which can be done on a finger or via a blood draw, is the same.


As previously stated, warfarin is reactive and has a narrow therapeutic index. It prevents clotting by inhibiting vitamin K, and in individuals who irregularly consume vitamin K foods, like leafy green vegetables, that can make the drug less effective. Alternately, many types of medicines, and even viruses or infections, can amplify warfarin’s effects and put people at risk for excessive bleeding. The amount of things that can influence the medication's effectiveness means that there is a undeniable connection between warfarin and INR readings. Giving the medication without testing is medically negligent because there is no way to tell if too much or too little is being used.

Most people discover this connection in regular visits to a lab or anti-coagulation clinic. Often, clinics are preferred because medical staff are on hand to make recommended adjustments to dosage immediately through PT/INR interpretation. Of course, clinics are not the only place where testing is done; people can be tested in hospitals, at doctor’s offices, or in their own homes with a special self-testing apparatus. Home tests may be a safer option if a person has shown previous volatility in PT readings.

In the most essential way, warfarin and INR readings relate to each other because the results provided by the test tell doctors whether patients need increases or decreases in their dose. Individuals need blood clotting times to be in a specific range, such as 2.0-3.0 seconds, which is defined by the medical condition being treated. If blood clots in less time, a higher dose of warfarin is given. Longer clotting times translate to a dose reduction.

The first few months of warfarin therapy are often the most critical period. An individual’s response to the drug is unpredictable, and medication levels must increase slowly. This means patients may require numerous INR readings. As response to the drug stabilizes, tests may be conducted less often. This could change if a patient gets ill, needs new medication, or makes sudden and dramatic dietary changes.


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