A new method of calculating the correct amount of warfarin - an anti-blood clotting medicine given to patients at risk of heart attacks or strokes - is believed to be more accurate that the current practice of adjusting the standard dosage through trial and error. The right amount of warfarin depends on each individual patient and giving a patient too little of the drug could leave them at risk of stroke or heart attack, but too much can lead to fatal bleeding.
A team of US scientists at Stanford University, California, used genetic information from over 4,000 patients to devise a pharmacogentic algorithm to take into account each individual's genetic variations to predict the response to the drug. The study was funded by the National Institutes of Health (NIH) and other medical organisations with the results being published in the New England Journal of Medicine this month. When the algorithm was tested it was found to be accurate in setting the correct dose in around one out of three patients using warfarin.
The standard started dosage of warfarin is five milligrams daily but some patients may require up to ten times this dosage for the drug to work effectively. The algorithm proved accurate in setting the dosage of patients requiring low and high dosage rather than those requiring an intermediate dosage. The researchers emphasised the limitations of the method implying the results do not guarantee patient safety: 'Our analysis does not address the issue of whether a precise initial dose of warfarin translates into improved clinical endpoints', the researchers said.
Janet Woodcock and Lawrence Lesko, of the FDA's Centre for Drug Evaluation and Research, said in an editorial accompanying the article that 'a better understanding of individual differences in the response, either positive or negative, to medicines should be an overarching goal for pharmacotherapy over the next decade. Pharmacogenetics has the potential to increase benefit and reduce harm in people whose drug responses are not 'average'.'
The NIH is launching a widespread testing programme before introducing the method, but commentators say it could be a couple of years before it is applied to patients.
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