Blacks treated with a drug-coated stent to open clogged heart arteries are nearly three times more likely to develop a life-threatening blood clot than whites, U.S. researchers said on Monday.

They said being black was the strongest predictor of stent thrombosis -- blood clots that can occur after a stent is implanted -- even after considering other known risk factors such as diabetes, high blood pressure or kidney problems.

Physicians and patients need to know that African-Americans are at a higher risk of developing stent thrombosis, which is associated with heart attack or death, said Dr. Ron Waksman of Washington Hospital Center and professor at Georgetown University, whose study appears in the journal Circulation.

Stents, made by companies such as Boston Scientific, Abbott Laboratories, Medtronic Inc and Johnson & Johnson, are wire-mesh coils used to prop open arteries narrowed by fatty deposits called plaque.

Many models release drugs over time that help prevent scar tissue from building up and blocking the artery.

Sanofi-Aventis' anti-clotting drug clopidogrel or Plavix is typically prescribed after a stent is implanted to keep patients from developing blood clots, but some patients, including some blacks, get them anyway.

Waksman and colleagues examined rates of stent thrombosis in nearly 1,600 blacks and 5,600 nonblacks given a drug-eluting stent. They checked for factors such as socioeconomic status or other health conditions that could explain higher rates of blood clotting.

After adjusting for different rates of diabetes, high blood pressure, kidney problems and how faithfully patients took their anti-clotting medication, they found being black was the biggest predictor of whether a patient would have a blood clot after getting a stent.

Blacks were nearly three times as likely to experience clotting as nonblack patients, with 1.71 percent of black patients having a clot after 30 days, compared with 0.59 percent of nonblacks.

The rate jumped to 3.67 percent for blacks after three years, compared with 1.25 percent of nonblacks.

And nearly a quarter of black patients who had been in the study were dead three years later -- for any reason -- compared with 13 percent of the nonblack patients.

Because blacks in the study were actually more likely than whites to take Plavix, and socioeconomic status was not a major predictor of blood clots, the researchers said being black is likely an independent predictor of who will develop a blood clot after getting a stent.

Because our analysis adjusts for traditional variables associated with racial disparities in health care, further mechanisms such as genetic polymorphisms and responsiveness to antiplatelet therapy must be pursued, Waksman and colleagues wrote.

Some people are not able to convert Plavix to its active form, and the researchers said blacks may be more prone to this problem.