Atlanta ? People taking warfarin, a leading blood thinner to prevent clots that cause heart attacks and strokes, soon may have a better way to get the tricky dose right.

A new formula that includes gene testing proved much better at setting the ideal dose than what doctors do now: Give a standard amount and adjust it by trial and error. The formula was tested in a large international study, which found the usual approach gets it wrong about half the time.

About 4 million Americans take warfarin, also known as Coumadin, the top-used blood thinner worldwide. It could be used even more, but doctors have worried about the all-too-common risks to patients if they get the dose wrong. Too little means a risk of stroke and too much can mean fatal bleeding.

The new study is one of the first to show genetic testing can be used to prevent dosing problems, experts said.

A new experiment will soon test the gene study’s results in a more rigorous way. Most patients will likely have to wait at least a few years before genetic testing becomes a common factor in warfarin dosing, some experts said.