Tests predict sensitivity to bloodthinner warfarin

BOSTON (Reuters) - Researchers have identified a new way to predict who is likely to develop dangerous bleeding or other serious reactions from taking the anticoagulant warfarin.

They found that one of two genes already implicated in dangerous responses to warfarin can predict who will have a more immediate reaction.

Currently, the U.S. Food and Drug Administration recommends that doctors start with lower doses of the drug for people who have certain variants of VKORC1 and CYP2C9.

Such tests are important. Some 2 million Americans take warfarin after a heart attack, stroke or major surgery and its effect can vary widely from person to person.

Doses must be carefully adjusted to prevent blood clots without causing excessive bleeding.

Dr. Ute Schwarz and colleagues at Vanderbilt University School of Medicine in Tennessee looked at the DNA of 297 patients who had just started taking warfarin, often sold by Bristol-Myers Squibb Co. under the brand name Coumadin.

Doctors looking for which genes might predict response to warfarin had been focusing on a gene called CYP2C9.

But Schwarz's team found that variations in a different gene called VKORC1 were key for predicting how people would react to their initial doses of warfarin.

Only later does knowledge of the CYP2C9 profile become useful, they wrote in their report, published in the New England Journal of Medicine.

"Both ... had a significant influence on the required warfarin dose after the first two weeks of therapy," they wrote.

"The first months of anticoagulant treatment are particularly problematic, since the safe and effective dose for an individual patients is not known and is determined empirically," they added.

"The administration of warfarin is tricky because of the drug's narrow therapeutic range and the large variations in dose requirements from one patient to another," Drs. Susan Shurin and Elizabeth Nabel wrote in a commentary in the Journal.

They said such studies will help move the health care system toward more tailored medicine.

"We are still a long way from the day when a patient presents a DNA 'chip,' a key-chain tag bearing the patient's electronic health record, and an insurance card to a physician and gets a dose of personalized medicine," they added.

(Editing by Maggie Fox and Xavier Briand)