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Warfarin (Coumadin®) is a blood thinner given to people at high risk for the formation of blood clots due to conditions such as deep vein thrombosis, heart valve disease or replacement, and irregular heart beat. The drug is also given to prevent recurrence of pulmonary embolism, heart attack and stroke.
If too little warfarin is prescribed, the threat of blood clots will remain. But if too much drug is given, uncontrolled bleeding can result. Add to this the fact that “too little” and “too much” can vary significantly between people, and it’s no wonder that complications from incorrect warfarin dosing are one of the most common reasons for emergency room visits due to adverse drug reactions.
In 2007, the FDA updated the labeling information for warfarin to include information about how variations in two genes, CYP2C9 and VKORC1, can impact a patient’s optimal dose of the drug. But the FDA did not make specific dosing recommendations at that time.
Now the FDA has updated warfarin’s label once again, this time specifically saying that a “patient’s CYP2C9 and VKORC1 genotype information, when available, can assist in selection of the starting dose.” The agency also provides initial dosage recommendations for patients with different variant combinations. The FDA does not, however, require that genetic testing be done before prescribing warfarin.
The warfarin label lists non-genetic factors that can impact warfarin dosing, including age, race, body weight, sex, other medications and other medical problems. These factors are accounted for in the dosing recommendations given in the label.
The Centers for Medicare and Medicaid Services, the federal agency that makes decisions about what Medicare, Medicaid and the Children’s Health Insurance Program will cover, decided last year that there was not sufficient evidence to justify covering genetic testing before warfarin administration for Medicare beneficiaries. They did say, however, that they would pay for testing in patients who are part of clinical studies aimed at gathering more data on how outcomes are influenced by the availability of genetic information.
Many physicians are still not comfortable incorporating genetics into their prescription decisions. The Pharmacogenomics Reporter quoted Janet Woodcock, director of the FDA’s Center for Drug Evaluation and Research, as saying, “Doctors tell us all the time, don’t tell us about VKORC1, just tell us what the right dose is for patients,” at a recent conference.
The American Medical Association has produced a booklet to help doctors understand the science behind genetics and warfarin dosing.
23andMe Health Edition and Complete Edition customers can view their data for the genetic variations in CYP2C9 and VKORC1 considered in the FDA labeling update in the Warfarin (Coumadin®) Sensitivity Drug Response Report.
A downloadable PDF version of the Warfarin Sensitivity report is available to help customers share their results with their doctors.
Only a physician can determine whether warfarin is the right medication for a particular patient and at what dosage it should be given. The information contained in 23andMe Drug Response reports should not be used to independently establish a drug regimen or abolish or adjust an existing course of treatment.
The Spittoon previously reported on one of the studies that contributed to the FDA’s new labeling decision.