A new study has found that a blood protein that reflects how well the kidneys work can predict an increased risk of heart attacks, stroke and death among the elderly.

The protein, called cystatin C, was a better indicator of kidney function than creatinine, the blood protein that doctors have been using for decades to assess renal function, researchers said.

Findings from the Cardiovascular Health Study, which included 1,501 local participants, show that as blood levels of cystatin C rise, so does the risk of death. Creatinine could not predict risk as well as cystatin C.

"This is the first study of its kind," said lead investigator Dr. Michael Shlipak of the Veterans Affairs Medical Center in San Francisco. "We think we've made a big step forward with this study."

The findings were published Thursday in the New England Journal of Medicine.

Researchers measured cystatin C and creatinine levels in serum samples that were drawn from more than 4,600 people in 1992 or 1993 and then stored. The participants, who were 65 or older when they joined the study, were monitored for an average of eight years.

Cystatin C and creatinine are cleared from the body by the kidneys, so higher levels of the proteins indicate the kidneys are not working as well as they should. Typically, doctors measure creatinine to assess kidney function.

"Creatinine is what's been used for 100 years," Shlipak said. "It's used everywhere by everybody."

But the researchers found creatinine only identified an increased risk of death among the 10 percent of participants with the very highest levels, or "the tip of the iceberg" as Shlipak put it.

Cystatin C measures drew a far different picture. As protein levels rose, the risk of death steadily increased. The 20 percent of participants with the highest levels were twice as likely to die from all causes and were 50 percent more likely to have a heart attack or stroke than those with the lowest levels.

"We were just blown away by these findings," Shlipak said. "What it tells me is that kidney function is far more important as a determinant of prognosis and risk of having a cardiovascular event than we could possibly have conceived."

How kidney function influences cardiovascular disease is not well understood, but blood pressure elevations, inflammation and other factors probably play a role, Shlipak said.

Cystatin C could be particularly useful in assessing kidney function in the elderly, said investigator Dr. Anne Newman, a geriatrician at the University of Pittsburgh's medical school.

"In older people, the creatinine level goes up with kidney failure, but it goes down when you lose muscle mass," she explained. "If you're getting older and sicker, your creatinine may stay rock stable when in fact your kidney function is deteriorating."

Creatinine is a good marker for people with known kidney disease, but cystatin C might let doctors detect problems before symptoms appear, Newman said.

But without more research, she said, "we still don't know if we can do anything to fix it."