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; however, 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.
Warfarin Sensitivity Testing
Warfarin Sensitivity Testing
Also Known As
Warfarin Sensitivity or Response Genotyping
Formal Name
CYP2C9 Genotype
VKORC1 Genotype
This article was last reviewed
on
This article waslast modified
on September 18, 2020.
At a Glance
Why Get Tested?
When To Get Tested?
A healthcare practitioner might order this test prior to prescribing warfarin for you or 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, a swab rubbed on the inside of your check (buccal swab), or you spit saliva into a clean container
Test Preparation Needed?
None
Accordion Title
Common Questions
-
How is the test used?
Warfarin sensitivity testing (genotyping) may be used to help determine your likely sensitivity to warfarin and to help select appropriate doses. Warfarin genotyping primarily identifies variations in the VKORC1 gene and the CYP2C9 gene. (See "What is being tested?" for more details on these genes.)
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 you taking warfarin for the first time, but it may also be ordered when you have had difficulties with achieving a stable anticoagulant ("blood-thinning") effect or have experienced either excessive blood clotting or bleeding while taking warfarin.
-
What does the test result mean?
Warfarin sensitivity testing will identify the CYP2C9 and VKORC1 genetic variants (i.e., genotype) present. Laboratories most commonly look for the CYP2C9 variant alleles *2 and *3.The results must be interpreted carefully to help determine if you are likely to need a relatively lower dose of warfarin. These results are often interpreted by a specialist. Some laboratories also test for other genes that may be involved in warfarin sensitivity and dose optimization (e.g., CYP4F2). The laboratory should provide information about any additional genes included in a test.
If you have one or more variant CYP2C9 or VKORC1 gene copies, you are more likely to need a lower dose of warfarin. The amount needed will depend on the number and type of genetic variants present but will also greatly depend upon other factors, including your health, age, sex, diet, and other medications.
This test detects only the most common genetic variants in CYP2C9 and VKORC1. You 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.
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.
-
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.
-
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).
-
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.
-
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. CYP2C9 is involved in metabolism of many other drugs.
-
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 healthcare practitioner.