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A team of scientists lead by researchers at Intermountain Medical Center has created a formula for using genetics to customize doses of a widely used, but dangerous, blood thinner.

The formula, a mathematical model combined with patient genetic and health data, helps doctors administer the popular blood thinner warfarin more accurately and safely, say the authors.

Results are published in the February edition of the journal, Thrombosis and Haemostasis.

Warfarin is prescribed to more than two million Americans each year. But patient response is variable.

If patients get too much of the drug, they can develop uncontrolled bleeding. Too little and they can get blood clots or have a stroke.

"Physicians usually test a patient's blood once or twice in the first week after starting warfarin to see if it's clotting properly," said Benjamin Horne, director of genetic epidemiology at the center's Heart Institute and the study's lead author. "But we've shown that because of genetic variation, patient response to warfarin continues to change after the first week and dosing may need to change, too."

Physicians and scientists at 13 health centers spanning the U.S. Europe, and Asia, tracked 1,342 patients on days six through 11 after beginning treatment with warfarin. The team considered factors such as individual body mass index, medication usage, age, gender, and whether or not the patients carried two genes known to influence the way warfarin affects the body.

From those data, Horne and colleagues generated an algorithm, which they tested on 342 patients.

Other centers involved in the study were Washington University in St. Louis, Harvard University, the University of Utah/ARUP, Kaiser Permanente Colorado, the Marshfield Clinic, Vanderbilt University, the University of Alabama, Uppsala University (Sweden), the Karolinska Institute (Sweden), the University of Liverpool (United Kingdom), Inje University (South Korea), and Mahidol University (Thailand).