Wed Feb 18, 2009
BOSTON (Reuters) - A new system for predicting the proper dose of the blood thinner warfarin could help doctors do a better job of preventing dangerous blood clots and serious bleeding, researchers reported on Wednesday.
Some patients needed more than twice the weekly dose than others did, the study showed, and the new test better predicted who needed more and who needed less.
Correct dosing of warfarin is critical -- the drug, sold under brand names including Coumadin, is based on rat poison and can prevent blood clots. But too high a dose and a patient can bleed to death, while too low a dose can cause strokes or heart attacks as clots form.
The international research team led by Dr. Teri Klein of Stanford University in California took clinical and genetic information from 4,043 patients to create a computer algorithm predicting who would need a higher or lower dose.
The new approach is called pharmacogenetics -- using patient's genetic variations to predict their response to drugs.
Writing in the New England Journal of Medicine, the researchers said they tested this algorithm in 1,009 other people.
"The ... algorithm accurately identified larger proportions of patients who required 21 mg of warfarin or less per week and of those who required 49 mg or more per week," they wrote.
About 30 million prescriptions for the drug are written in the United States each year and repeated testing to assess clotting time is needed to find the right dose. The safest dose for one person may be 10 times higher than the best dose for someone else.
The National Institutes of Health is about to launch a large-scale test with 1,200 volunteers to see if the new system works better.
"This has the potential of getting one genomic test up front and then starting off closer to the final dose early on, decreasing the amount of time and nuisance for repeated testing to get to the right dose," Dr. Jeremy Berg, director of the National Institute of General Medical Sciences, which sponsored the study, said in a telephone interview.
"Pharmacogenetics has the potential to increase benefit and reduce harm in people whose drug responses are not 'average'," The U.S. Food and Drug Administration's Dr. Janet Woodcock and Lawrence Lesko wrote in a commentary.
(Reporting by Gene Emery; Editing by Maggie Fox and David Wiessler)