A screening test for Prostate cancer taken by millions of men every year is not terribly accurate and not even the best result ensures that a man is cancer-free, experts said on Tuesday.

They found the standard prostate-specific antigen, or PSA test, produces many false positives and false negatives - meaning some men who think they do not have cancer actually do, while others may undergo uncomfortable biopsies only to learn there is no tumor after all.

“Patients have assumed, ‘My PSA is below 4. It’s normal. I have no risk’,” Dr. Ian Thompson of the University of Texas Health Science Center at San Antonio, who led the study, told a news conference.

In fact, some men with PSA levels of 1 had Prostate cancer, his study showed. Others with higher PSA levels did not have prostate cancer.

If all men got biopsies when PSA reached 1.1, more than 80 percent of all prostate cancers would be detected, Thompson said. But 61 percent of men who got biopsies would turn out not to have cancer.

A cutoff of 2.6 would detect only 40.5 percent of cancer cases.

This could explain why some men die of Prostate cancer despite intensive screening programs, the researchers said.

“This is going to require a re-education not just of patients but of physicians,” Thompson said. “What should men do? Our take home message ... they should consider the risk factors.”

Men whose fathers or brothers had prostate cancer, black men and others have a higher risk than the general population, for instance, he said.

RANGE OF RISK

“PSA, like blood pressure, like cholesterol, like many other tests, shouldn’t be considered to be ‘normal’ or ‘abnormal’ but should be considered as showing a range of risk,” Thompson said.

For their study, published in the Journal of the American Medical Association, Thompson and colleagues examined more than 5,000 men who took part in a larger study that showed the drug finasteride, sold by Merck and Co. under the brand name Proscar, could help prevent Prostate cancer in some men.

All the men in the latest study got a placebo instead of finasteride and they were watched for seven years. At the end, all of them got biopsies, regardless of PSA level.

Nearly 22 percent of the 5,000 men were found to have Prostate cancer, either during the seven years or at the end.

The good news was that PSA was accurate in detecting high-grade prostate cancers - those more likely to spread a kill a patient.

Prostate cancer affects more than 200,000 men a year in the United States and will kill 29,000 in 2005, according to the American Cancer Society.

It is usually a slow-growing cancer and older patients often can simply keep an eye on it because they are likely to die of something else first. But there is no good way to tell who can safely get away with “watchful waiting.”

An American man has a 17 percent lifetime risk of Prostate cancer, but only a 3 percent risk of dying from it.

In 2001 about 75 percent of U.S. men 50 and older reported having at least one PSA, and 54 percent say they have it measured regularly.

In a separate study, researchers at the American Cancer Society found that men who use aspirin and other nonsteroidal anti-inflammatory drugs or NSAIDs have a slightly reduced risk of prostate cancer.

But the society said their findings from a study of 70,000 men do not yet justify taking analgesics primarily to prevent Prostate cancer.