WASHINGTON – A highly anticipated study released Sunday has produced powerful evidence that a simple blood test can spot seemingly healthy people who are at increased risk for a heart attack or stroke and that giving them a widely used drug offers potent protection against the nation’s leading killers.

In findings that could transform efforts to prevent cardiovascular disease, the study of nearly 18,000 volunteers in 26 countries found that a cholesterol-lowering statin, or drug, slashed the risk of those flagged by the test by about half – even if their cholesterol was normal.

“The potential public health benefits are huge,” said Paul Ridker of the Brigham and Women’s Hospital in Boston, who presented the findings at a meeting of the American Heart Association in New Orleans. “It really changes the way we have to think about prevention of heart attack and stroke.”

The test measures a bodily reaction known as inflammation, reinforcing an increasingly accepted theory about the underlying biology of heart disease, which kills about 450,000 Americans each year.

“It’s a breakthrough study,” said Steven E. Nissen of the Cleveland Clinic, who was not involved in the research. “It’s a blockbuster. It’s absolutely paradigm-shifting.”

Several leading authorities predicted the findings would prompt many doctors to start routinely screening middle-aged patients for inflammation with the $20 test and begin prescribing the statin used in the study, or one of the less expensive generic versions, to those who get worrisome results. All such drugs reduce inflammation.

“This takes prevention to a whole new level,” said W. Douglas Weaver, president of the American College of Cardiology. “Yesterday you would not have used a statin for a patient whose cholesterol was normal. Today you will.”

Some experts, however, worried about rushing millions of healthy people onto powerful drugs.

“This would be a huge expansion of the boundaries of drug therapy,” said Mark Hlatky of Stanford University, who wrote an editorial that will accompany a paper describing the findings in the New England Journal of Medicine. “I think we need to be careful before we radically change what we do. Nothing is risk-free.”

The traditional model for how heart attacks and strokes occur is that high cholesterol causes fatty buildups to slowly accumulate inside arteries supplying blood to the heart and brain, but about half of all heart attacks and strokes occur in people whose cholesterol is normal. That has raised questions about what other factors may be involved and how more deaths could be prevented.

“This has been the puzzle,” Ridker said. “How do we identify these people and prevent disease in these folks?”

Evidence has been building that inflammation, part of the body’s defense against infection and injuries, may play a crucial role by causing the most vulnerable plaques inside arteries to rupture, triggering blood clots that finally block blood flow. The blood test detects inflammation by measuring a substance in the blood called C-reactive protein, which statins drive down. But it has remained far from clear how important or useful that information was.

In 2003, Ridker and his colleagues started prescribing either 20 milligrams of the statin Crestor or an inert placebo daily to 17,802 middle-aged and elderly men and women who had what are considered safe cholesterol levels but high CRP – 2 milligrams per liter of blood or above.

The researchers planned to follow the subjects for five years, but an independent panel monitoring the study stopped the trial in March after an average follow-up of less than two years, concluding that the benefit was so striking that it would unethical to continue withholding the real drug from those taking the placebo. But no details were released at the time.

Compared with those getting the placebo, those taking Crestor were 54 percent less likely to have a heart attack, 48 percent less to have a stroke, 46 percent less likely to need angioplasty or bypass surgery to open a clogged artery, 44 percent less likely to suffer any of those events and 20 percent less likely to die from any cause, the researchers reported Sunday.

“We were both shocked and elated,” Ridker said.

The numbers were relatively small – for example, 31 heart attacks in the statin group versus 68 in the placebo group, reducing the rate from .37 to .17 per 100 person years (or 100 people living one year). But the relative reduction in risk was about double that seen in any previous study evaluating a statin, which are among the most widely prescribed medications. The data are also the first to offer clear evidence of a benefit for women and minorities as well as men.

“These are very, very dramatic findings,” said Elizabeth Nabel, director of the National Heart, Lung and Blood Institute, noting that two other studies presented at the meeting support the value of CRP testing.

Experts will review the data before deciding how to revise federal guidelines on CRP testing and statin treatment, Nabel said. But she and others said the results will likely have a significant impact on how doctors try to prevent heart disease.

Ridker said that one of his colleagues predicted that an estimated 250,000 heart attacks, strokes, angioplasties or deaths from heart attacks could be prevented in the United State alone over five years.

Although there has been concern about the safety of Crestor, the researchers found no signs of significant risks. The study was funded by AstraZeneca, which makes Crestor, but the company had no influence over the analysis, Ridker said. He and his hospital receive royalties from the high-sensitivity CRP test, but other researchers said that was no reason to doubt the findings.

It remains unproven that other statins would produce similar benefits, but other statins also reduce CRP and experts said it was likely that they would be beneficial, though the magnitude of the benefit may be smaller.