Patient Test Information

Warfarin Sensitivity Testing

Also known as:

Warfarin Sensitivity Genotyping

Formal name:

CYP2C9 Genotype; VKORC1 Genotype

Related tests:

PT and INR, Pharmacogenetic Tests

Board approvedAll content on Lab Tests Online has been reviewed and approved by our Editorial Review Board.

Why Get Tested?

To determine whether you have CYP2C9 and/or VKORC1 genetic variations and are likely to need lower, or less commonly, higher than average doses of the "blood-thinning" drug (anticoagulant) warfarin

When to Get Tested?

A health practitioner might order this test prior to prescribing warfarin for you; a health practitioner may sometimes order it when you are being treated with warfarin and have had bleeding or clotting episodes or dosing difficulties

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


How is it used?

Warfarin sensitivity testing (genotyping) may be used to help determine someone's likely sensitivity or, less commonly, resistance to warfarin and to help select appropriate doses. Warfarin (Coumadin®) is an anticoagulant, a drug that is prescribed to help prevent inappropriate blood clotting (thrombosis and thromboembolism) in people who are at risk, or to help keep an existing blood clotting condition from getting worse.

Warfarin genotyping identifies variations in certain genes.

  • The VKORC1 gene codes for the protein that warfarin targets. A variation in this gene may result in a protein change that may be more sensitive or less sensitive (or resistant) to the anticoagulant effect of warfarin. Depending on the variant present, the person tested may need a lower or higher initial dose of warfarin.
  • The CYP2C9 gene codes for a protein involved in the breakdown (metabolism) of warfarin. People with variations in this gene may metabolize warfarin more slowly, resulting in slower clearance of the drug and accumulation in the body over time. They may require a lower dose of the drug.

This testing may sometimes be used to help determine an appropriate dose for a person who has previously taken warfarin and either experienced inappropriate clotting or bleeding episodes while on warfarin or had to go through many dose adjustments to reach a stable anticoagulant ("blood-thinning") effect.

CYP2C9 and VKORC1 genetic testing is not widely used at this time. Although studies have shown that these genes contribute to a person's sensitivity to warfarin and can account for a significant percentage of the person-to-person variation in warfarin doses, there is not a consensus on the need for the testing yet.

When is it ordered?

Warfarin sensitivity testing may be ordered prior to a person taking warfarin for the first time but may also be ordered when a person has had difficulties with achieving a stable anticoagulant ("blood-thinning") effect or has experienced either excessive blood clotting or bleeding.

Not everyone who is prescribed warfarin will have this test done. At present, there is no consensus on the usefulness of this test and it is not yet widely accepted.

What does the test result mean?

Warfarin sensitivity testing will identify the CYP2C9 and VKORC1 genetic variants (i.e., genotype) present for the person tested. The results must be interpreted carefully to help determine if the person is likely to need a relatively lower or higher dose of warfarin. These results are often interpreted by a specialist.

Those who have one or more variant CYP2C9 or VKORC1 gene copies are more likely to need a lower or higher dose of warfarin. The amount needed will depend on the number and type of genetic variants present but will also greatly depend upon a person's other factors, including health, age, sex, diet, and other medications.

This test detects only the most common genetic variants in CYP2C9 and VKORC1. A person may have a rare variant, resulting in a negative test result, but may still be more sensitive or resistant to warfarin.

Is there anything else I should know?

Some major health organizations support warfarin sensitivity testing, while others would like to see more data showing that the use of the testing actually has an impact on the health of the patient, such as a decrease in the number of clotting or bleeding events experienced by those on warfarin. Currently, there are studies underway that may provide additional data on health outcomes in the next few years, and that is likely to help determine the clinical usefulness of testing.

The U.S. Food and Drug Administration (FDA) has created changes to the warfarin label to provide information about the usefulness of CYP2C9 and VKORC1 genetic testing and to provide guidance on warfarin doses based upon testing results.

Whatever the initial dosage of warfarin, each person taking warfarin will still need to be monitored with the PT/INR test and all subsequent dose adjustments will be made by taking these test results into account.

Many drugs interact with warfarin and can slow the metabolism of warfarin. Make sure all of your healthcare providers are aware of all the medications you are taking.

Warfarin is prescribed to those at risk for inappropriate clotting, including those who have had a previous blood clot; those with excessive clotting disorders, atrial fibrillation, artificial heart valves; and those who have undergone orthopedic surgeries.

What is being tested?

Warfarin (Coumadin®) is a "blood-thinner" (anticoagulant), a drug that is commonly prescribed to help prevent inappropriate blood clotting (thrombosis and thromboembolism) in individuals who are at risk or to help keep an existing blood clotting condition from getting worse. Warfarin sensitivity testing helps determine if a person may be more sensitive (i.e., require lower doses) or, less commonly, resistant (i.e., require higher doses) to treatment with warfarin because of his or her genetic makeup. This test detects common genetic variations in two genes, CYP2C9 and VKORC1. A health practitioner may sometimes order this test to help select appropriate doses of warfarin and/or to achieve appropriate dose levels more quickly.

Warfarin can be a challenging drug to administer and monitor because its levels can be affected by many different factors and it has a narrow window in which it is effective. If a person is given too little drug, he or she may be at risk of forming a blood clot; if given too much drug, he or she may be at risk of moderate to severe bleeding episodes. The range between too much drug and too little drug is small and it varies significantly from person to person.

Historically, health practitioners have given warfarin doses based upon age, weight, and sex, and then have monitored and adjusted dosages using frequent PT/INR (prothrombin time/international normalized ratio) tests to determine the medication's anticoagulant ("blood-thinning") effect. Health practitioners then use periodic PT/INR tests to ensure that a stable dose with sufficient anticoagulation has been achieved. Although this dosing strategy is still commonly used, it can take as long as several weeks.

Genetic testing for warfarin sensitivity may help to shorten the time it takes for some people to reach a stable dose; that is, to achieve adequate "blood-thinning" without bleeding episodes. Most often, it is determined that people are more sensitive to warfarin and may need smaller doses; however, there are some people who are less sensitive and may need larger doses. The reason for the relative sensitivity can be, at least in part, due to the individual's genetic makeup and may involve two genes:

  • The VKORC1 gene codes for the production of a protein (VKOR) that is involved in the production of functional forms of certain clotting factors (II, VII, IX, X) that allows them to participate in the clotting process. Warfarin prevents the action of VKOR. Therefore, a variation in VKORC1 may cause someone to be more or less sensitive (or resistant) to warfarin.
  • The CYP2C9 gene codes for the production of a protein (CYP2C9) involved in the breakdown (metabolism) of warfarin. People with CYP2C9 variations may not be able to get the drug out of their system as efficiently as others. The people who clear the drug more slowly may require lower (sometimes significantly lower) doses of warfarin.

Each person receives one copy of each of their genes from their mother and one copy of each of their genes from their father. Thus, each of the CYP2C9 and VKORC1 genes is present in the body as two inherited gene copies (alleles). Any person could have both copies of a gene without any variants (wild-type); one copy without variants and one copy with a variant (heterozygous); both copies with the same variant (homozygous); and both copies with different variants ("compound heterozygous"). The combination of CYP2C9 and VKORC1 gene copies that a person has can determine the overall effect of warfarin and how rapidly it is metabolized and cleared from the body.

Warfarin sensitivity (or warfarin genotype) testing determines whether CYP2C9 and VKORC1 gene variants are present. Careful interpretation of the results can help the health practitioner decide the appropriate doses of warfarin for the person tested.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

  1. If I have genetic testing done, will I still need to be monitored with PT/INR tests?

    Yes. Regardless of your genetic makeup, your response to warfarin therapy will still need to be monitored with regular PT/INR tests. This is because your individual degree of anticoagulation ("blood-thinning") can be affected by other factors such as diet, age, weight, other medications and state of health, which can change over time.

  2. Can warfarin sensitivity testing be done in my doctor's office?

    No, it requires specialized equipment to perform and expertise to interpret. It is not offered by every laboratory and may need to be sent to a laboratory outside of your hospital (i.e., reference laboratory).

  3. Will I need to have this test done more than once?

    No. This test determines information about the genes you have inherited. This information does not change so you will only need to be tested once.

  4. Should I tell other doctors about my warfarin sensitivity?

    Yes, and you should always tell them that you are taking warfarin. The use of warfarin and your sensitivity to warfarin is important information for the medical professionals (including dentists) that you see. It can have an effect on your treatment options.

  5. Should my other family members be tested for warfarin sensitivity?

    This is something to discuss with your healthcare provider and family. It is not generally indicated unless a family member is also going to be taking warfarin, but having a family member who has warfarin sensitivity is important information to keep in mind and to share with your health practitioner.