The approval of the test comes a month after warfarin, sold under the brand name Coumadin and in generic forms, became the first widely used drug to include genetic testing information on its label. The information can help doctors determine how best to prescribe the drug.

An estimated one-third of patients process the drug differently than do most others, exposing them to a higher risk of bleeding. Research suggests that most of that sensitivity is due to variations in two genes. The new test, made by Nanosphere Inc. of Northbrook, Ill., can detect some of those variants.

One of the genes produces an enzyme that helps the body metabolize warfarin and other medicines; the second produces the blood-clotting protein that warfarin blocks.

The test won't replace the need to do regular blood tests to ensure patients are receiving the correct dose of the drug. Tailoring the proper dosage remains largely a matter of trial and error, with a patient's age, weight, diet and other prescription drug use all playing roles.

The stakes are high: Patients taking too much warfarin can bleed to death. If people take too little of the drug, it can fail to protect them from deadly blood clots and stroke. After insulin used by diabetics, warfarin sends more people to the emergency room than any other drug.

Health officials hope genetic testing can reveal which patients may require less of the drug and lead doctors to recommend an initial dose closer to the lower end of the scale.

FDA economists have estimated the genetic testing could prevent 85,000 "serious bleeding events" and 17,000 strokes a year, according to a November 2006 study posted to the Web site of the American Enterprise Institute. The savings to the health care system could be $1.1 billion a year, though some people question that.

About 2 million people start taking the drug each year.