Offering the first evidence that prostate blood-screening truly saves lives, a new study concludes that routine use of the PSA test could prevent 27,000 of the 39,000 prostate cancer deaths in the United States each year.

The study, released last week, found that over an eight-year period, the test reduced deaths from prostate cancer by 69 percent by catching the disease early, while it was still treatable with surgery or radiation.The researchers predicted that if men started testing at age 50, when their risk of advanced disease is still small, it could practically eliminate the development of prostate cancer that reaches the deadly spreading stage.

The study was done by Dr. Fernand Labrie of Laval University in Quebec, who pioneered the use of hormone-blocking drugs as an alternative to castration in advanced prostate cancer. He presented the results at the annual scientific meeting of the American Society for Clinical Oncology.

Few tests are as controversial as the PSA, and the new study is unlikely to settle this, however.

The test looks for elevated levels in the blood of prostate-specific antigen, or PSA, a protein that rises in prostate cancer.

Although PSA testing has become a standard part of men's check-ups over age 50, some cancer experts question whether this is always a good idea, especially for men in their 70s and beyond.

The critics contend there is no good evidence that removing cancerous prostate glands reliably catches the disease before it has spread.

The cancer conference, the world's biggest, gave Labrie's study center stage as a plenary presentation, implying it considered this to be one of the most important on the program. At the same time, however, conference officials went out of their way to downplay it.

"I'm not yet convinced," said Dr. Derek Raghaven of the University of Southern California, who moderated a news briefing.

At issue were technical questions about how the study was con-duct-ed.

The study was launched in 1988, well before PSA testing became rou-tine.

Labrie's team randomly divided all 46,193 men in the Quebec City area between ages 45 and 80 into two groups. Half were invited by letter to have prostate cancer screening.

However, only 8,137 actually agreed to be tested. So the researchers compared their risk of dying with that of the 38,056 who were not screened.

Raghaven and others questioned whether those who actually showed up for screening - and those who stayed home - might have been influenced by factors such as whether they had a family history of the disease. If so, their cancer risks might be different, so a comparison would be invalid.

Labrie countered that the cancer death rate was the same in those randomly assigned to the comparison group and those who turned down screening.

Over eight years of follow-up, there were five prostate cancer deaths among the men getting regular PSA tests, compared with 137 in the larger non-screened group. This works out to a threefold greater risk in the men with no screening.

When applied to the entire U.S. population, Labrie calculated that regular PSA screening would save 27,050 lives a year.