One-third of patients receiving warfarin metabolize it quite differently than expected and experience a higher risk of bleeding. Research has shown that some of the unexpected response to warfarin depends on variants of two genes, CYP2C9 and VKORC1. The Nanosphere Verigene Warfarin Metabolism Nucleic Acid Test detects some variants of both genes.
"Today's action offers physicians the first FDA cleared genetic test for warfarin sensitivity, which is another step in our commitment to personalized medicine, said Daniel Schultz, M.D., director, FDA's Center for Devices and Radiological Health. With this test, physicians may be able to use genetic information along with other clinical information to treat their patients.
Warfarin can be a difficult drug to use because the optimal dose varies depending on many risk factors, including a patient's diet, age, and the use of other medications. Rapidly achieving the correct dose is important. Patients who receive doses that are higher than needed to correctly thin the blood are at risk of life-threatening bleeding. Those who receive doses that are too low may remain at risk of life-threatening blood clots.
Warfarin is the second most common drug, after insulin, implicated in emergency room visits for adverse drug events.
In August, FDA approved updated labeling for Coumadin, the brand name version of warfarin, explaining that people with variations of the genes CYP2C9 and VKORC1 may respond differently to the drug. Manufacturers of generic warfarin are adding similar information to their products' labeling.
Physicians and other health care professionals who prescribe warfarin regularly check to see if the drug is working properly by ordering a test called the PT or prothrombin time that evaluates the blood's ability to clot properly. The results are measured in seconds and compared with the expected value in healthy people, known as the International Normalized Ratio or INR.
The Nanosphere test is not intended to be a stand-alone tool to determine optimum drug dosage, but should be used along with clinical evaluation and other tools, including INR, to determine the best treatment for patients.
FDA cleared the test based on results of a study conducted by the manufacturer of hundreds of DNA samples as well as on a broad range of published literature. In a three site study, the test was accurate in all cases where the test yielded a result; 8 percent of the tests could not identify which genetic variants were present.
The new test was cleared for use on the Verigene System, a clinical laboratory test system. Both products are manufactured by Nanosphere Inc., Northbrook, Ill.
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