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Preventing blood clots: Is warfarin still right for you?

Using warfarin effectively is challenging—perhaps even more so than doctors have realized.

blood test warfarin preventing blood clots
Image: arun011 /Thinkstock

For more than 60 years, warfarin has been a mainstay for people with atrial fibrillation. Used properly, warfarin may prevent as many as 60% of strokes related to this common heart rhythm disorder each year. But warfarin is notoriously tricky to manage because blood levels have to be kept within a narrow range to avoid side effects such as unwanted bleeding.

As a result, warfarin users must get frequent blood tests (see “What is an INR?”) when they start the drug to fine-tune their dose. Once their INR values are consistently in the right range, most people scale back to testing every month or so. But a recent study suggests that people who take warfarin need to stay extra vigilant over the long term.

What is an INR?

Warfarin works by blocking the production of substances in the blood known as clotting factors. To use warfarin safely, you must take a test that measures how long it takes for your blood to clot, known as the prothrombin time test, or protime (PT).

Because the PT result depends on the specific chemicals used in the test, it is expressed as a standardized number called the international normalized ratio (INR). Based on the INR, your doctor can tell whether your warfarin dose is in a safe range—usually between 2.0 and 3.0, although it can vary from person to person depending on what condition warfarin is being used to treat. Many common drugs, foods, and dietary supplements affect warfarin, so the same dose may cause either too much or too little anti-clotting effect at different times.

An INR of 5.0 or higher means you’re at high risk of major bruising or bleeding. An INR of 1.5 or lower puts you at greater risk of developing a life-threatening blood clot.

Unstable doses?

The study, published in the Aug. 9, 2016, Journal of the American Medical Association, included 3,749 people with an average age of 75 who were taking warfarin. During the first six months, only about a quarter had INR values in the correct range 80% or more of the time. Of those with stable INRs during the first six months, only about a third remained stable over the following year. And among those with 80% or more INR values in range at baseline, 36% had one or more well-out-of-range INR values over the next year.

“This study shows that even people who we thought were stable on their warfarin dose might not be,” says Dr. Christian Ruff, a cardiologist at Harvard-affiliated Brigham and Women’s Hospital. Does this mean, as the study authors suggest, that warfarin users should consider switching to one of the newer anti-clotting medications, known as non–vitamin K antagonist oral anticoagulants (NOACs)? Not necessarily, says Dr. Ruff.

Since the first of these drugs (see “Warfarin alternatives”) became available in 2010, they have been slowly replacing warfarin. In fact, most people newly diagnosed with atrial fibrillation now receive a NOAC instead of warfarin, says Dr. Ruff. But for longtime warfarin users, most doctors have taken an approach of “if it ain’t broke, don’t fix it” to those who seem to be stable on the drug.

“This new finding suggests that we should be much more vigilant about making sure people who take warfarin are truly stable,” says Dr. Ruff. People in their 70s can experience many clinical changes over the course of a year, he notes, such as starting or stopping a medication, undergoing a procedure, or a getting new diagnosis. If you’re taking warfarin, be sure to stay in close contact with your physician if you experience any health-related changes, as you may need more frequent INR tests, he says.


Non–vitamin K antagonist oral anticoagulants (NOACs) target a different part of the blood clotting process than warfarin does. They include apixaban (Eliquis), dabigatran (Pradaxa), edoxaban (Savaysa), and rivaroxaban (Xarelto). These drugs prevent strokes just as well as warfarin but have far fewer interactions with drugs and foods and don’t require blood tests or frequent dose adjustments. NOACs also cause half as much brain bleeding as warfarin.

But there are downsides. NOACs don’t stay in your body very long, so it’s important to take them on schedule. So far, only dabigatran has an antidote (a drug that quickly stems uncontrolled bleeding), which is vital if you have an accident, for example. (A warfarin antidote has long been available.) Finally, NOACs cost about 30 times as much as warfarin, although your out-of-pocket cost depends on your insurance. All of these drugs may be available as lower-priced generics in a few years, however.

Posted by: Dr.Health

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