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American College of Medical Genetics responds to new FDA labeling decision for warfarin

August 27, 2007

On August 16, the Food and Drug Administration approved new labeling information for the blood-thinner warfarin, sold under the brand name Coumadin. The new label suggests that one can prescribe higher or lower doses that may be safer for patients with variations in two genes, CYP2C9 or VKORC1. These variations can be learned through genetic testing, which may be a useful tool in determining appropriate dosing levels for individual patients and lower risks of bleeding complications.

"The FDA labeling change for warfarin is the next step in the pathway toward incorporating genetics into the care of patients who need anticoagulation. The FDA has said that physicians should be aware of the role of genetics in warfarin dosing, but now the hard part is doing the necessary groundwork to determine how best to use genetic testing to improve care," said Marc S. Williams, MD, FACMG, ACMG Board Member and a co-chair of the expert group on "Pharmacogenetic Testing of CYP2C9 and VKORC1 Alleles for Warfarin Use." This group is completing a report on the use of pharmacogenomic testing for warfarin and is based on a rapid-ACCE (Analytic validity, Clinical validity, Clinical utility and Ethical, legal and social implications) Review on Genetic Testing and Warfarin Dosing.

Available online at www.acmg.net, this 74-page report is the most thorough review of the scientific and clinical evidence surrounding the use of genetic testing to guide dosing of warfarin and was undertaken by a multidisciplinary group convened in 2006 of clinical pharmacologists, doctors of pharmacy, clinical geneticists, physicians with expertise in the use of warfarin, pharmacoeconomists, and experts in evidence-based medicine. The purpose of the rapid-ACCE review was to provide an independent, systematic review of the evidence and to develop recommendations based on this evidence. The review indicated that the test itself is as accurate as most genetic tests and that there is strong evidence to support the relationship between the genetic variant(s) and the final dose of warfarin in patients. Further, "The report states that there are situations in which one should perform genetic tests when prescribing warfarin. In particular, it states in the conclusion that 'CYP2C9 and VKORC1 genotypes can reasonably be used as part of diagnostic efforts to determine the cause of an unusually low maintenance dose of warfarin or an unusually high INR (the test used to monitor warfarin) during standard dosing,'" said Michael S. Watson, PhD, FACMG, Executive Director of the American College of Medical Genetics who commissioned the study in 2006 with funding provided by the American College of Medical Genetics Foundation. However, the ACCE review and the work group noted that there are still significant gaps in our knowledge of the clinical utility and the balance between harm, benefit and cost. Resolving this is necessary before genetic testing becomes the standard of care for all patients undergoing anticoagulation with warfarin. There are also insufficient data about the impact of this testing on adverse events.

In response to the need for more information on implementing genetic testing into warfarin dosing, the American College of Medical Genetics will release a position statement in fall 2007 in the journal Genetics in Medicine which will provide the details of how the experts interpreted the evidence as to the use of these genetic tests to inform warfarin dosing.

"With 30 million Americans on warfarin and 2 million or more adverse side effects reported annually, the potential for genetic testing to minimize pain and suffering is tremendous. Genetics will continue to play an ever-increasing role in the prevention, diagnosis and treatment of disease," said Joe Leigh Simpson, MD, FACMG, President of the American College of Medical Genetics.

American College of Medical Genetics