Warfarin Response

Is Testing Right for Me? I Have My Test Results

Warfarin Response

What Is Warfarin?

Warfarin (Coumadin®) is the most commonly used prescription medication for preventing harmful blood clots from forming or from growing larger. Warfarin belongs to a class of drugs called anticoagulants, which simply means medications that prevent the blood from clotting. People often call these drugs "blood thinners."


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It is vital that our bodies can form blood clots to control bleeding. However, many medical conditions and inherited factors can make a person more likely to form abnormal blood clots. Abnormal blood clots are dangerous because they can block the flow of blood to parts of the body like the heart, lungs, or brain.

Some conditions that are treated with warfarin on a short-term or long-term basis include:

  • Irregular heartbeat
  • Heart valve replacement
  • Previous heart attack or stroke
  • Blood clot in a vein (venous thrombosis or deep vein thrombosis)
  • Blood clot in the lung (pulmonary embolism)
  • Certain orthopedic surgeries, such as knee or hip replacement
  • Inherited blood clotting disorders, such as Factor V Leiden

Anyone who has a higher-than-average risk for abnormal blood clots may be treated with warfarin.

Next: What Affects Warfarin Response?

Why are you considering warfarin response testing? Check only one at a time.

I may start warfarin therapy.

I'm taking warfarin now and I haven't had any problems with it.

I'm having problems with warfarin or I've had problems in the past. Problems could include blood tests that show your blood is too thin, bleeding (hemorrhage), or having a hard time finding the right dose.

None of these

Experts have mixed opinions about whether all people starting warfarin should automatically have warfarin response genetic testing. Most studies show that people with CYP2C9 or VKORC1 gene variants need lower doses of warfarin on average. However, large studies to show whether or not doctors can actually prescribe warfarin more safely when they know your genetic test results are still in progress. Most experts agree that testing may be useful for people who are at higher risk for complications from warfarin.

A committee of the National Institutes of Health has recommended that, if genetic test results are available, doctors should decide on warfarin dose by using a calculation that combines genetic test results and clinical factors (such as age and weight). The U.S. Food and Drug Administration (FDA) doesn't specifically recommend genetic testing before starting warfarin, but the drug label for warfarin includes a dosing table based on genetic test results.

If you have warfarin response genetic testing, the ideal time is before you start treatment. The risk of bleeding as a result of getting too much warfarin is highest in the first few weeks after starting warfarin, because no one can predict how your unique combination of factors will affect your response. During this time, your doctor measures your response with blood tests and adjusts your dose. Genetic test results could help your doctor more accurately estimate a safe and effective warfarin dose from the start.

If you have CYP2C9 and/or VKORC1 gene variants:

  • You can still take warfarin.
  • You may need to start warfarin therapy with a lower dose than usual.
  • You may need less warfarin over the long-term to keep your blood thinned the right amount.
  • You have a higher risk for bleeding or other complications when taking warfarin.
  • It may take longer to find a stable dose for you.
  • You probably need to be monitored more closely than usual.
  • CYP2C9 gene variants can affect how you respond to many other drugs. You should tell all your doctors and pharmacists about your test result. (See a list of Drugs Metabolized by CYP2C9)

If testing doesn't find any variants:

  • You don't have the most common genetic reasons for warfarin sensitivity.
  • Many other things affect how much warfarin you need and how you respond to it. Your doctor should take your genetic test results and other factors into account when figuring your starting dose.
  • You still need to be monitored regularly, as do all people taking warfarin.

Never stop taking warfarin or change your dose without first talking to your doctor.

Genetic testing for warfarin response may not be helpful for you. These results are most useful to figure out a safe starting dose or to explain complications from warfarin.

If your doctor has been able to keep your blood thinned to the right amount, your dose is right for you. Genetic test results won't change your dose in this case. Many things other than gene variants affect how much warfarin you need and how you respond to it. Everyone on warfarin should have his or her response measured regularly. Your regular dose is figured from your response to warfarin, not genetic test results.

Never stop taking warfarin or change your dose without first talking to your doctor.

Warfarin response testing could help explain why you've had problems with warfarin therapy. Experts don't agree that all people with warfarin complications need genetic testing. The American College of Medical Genetics says that:

"Although the routine use of warfarin genotyping is not endorsed by this work group at this time, in certain situations, CYP2C9 and VKORC1 testing may be useful, and warranted, in determining the cause of unusual therapeutic responses to warfarin therapy."

Until experts make clearer recommendations, the decision about testing is up to you and your doctor. Insurance companies may not cover the cost of testing.

If you have CYP2C9 and/or VKORC1 gene variants:

  • These results may explain your problems if you had bleeding, test results that showed your blood was too thin, or difficulty finding the right dose.
  • You can still take warfarin.
  • If you haven't found a dose that works well for you yet, these results could help your doctor find that dose. Your dose may be lower than usual.
  • You probably need to be monitored more closely than usual.
  • CYP2C9 gene variants can also affect how you respond to many other drugs. (See a list of Drugs Metabolized by CYP2C9) If you have a CYP2C9 variant, tell all your doctors and pharmacists about the result.

If testing doesn't find any variants:

  • You don't have the most common genetic reasons for warfarin sensitivity. (If you have non-Caucasian ethnicity, you may want to confirm that you were tested for the most common CYP2C9 variants for your ethnic background.)
  • Other factors may be affecting the way you respond to this drug. (See What Affects Warfarin Response? for more information)
  • You can probably still take warfarin, but you may need to be monitored more often.
Never stop taking warfarin or change your dose without first talking to your doctor.

Genetic testing for warfarin response may not be helpful for you. These results are most useful to figure out a safe starting dose or to explain complications from warfarin.

The VKORC1 gene variant is only known to be a factor in warfarin response. CYP2C9 is involved in how your body processes about 15% of prescribed drugs. If you think genetic testing for CYP2C9 may be important for predicting your response to drugs other than warfarin, talk to your doctor.