Many doctors are concerned that Canada's two best-known victims of prostate cancer, Preston Manning and Allan Rock, have been dispensing potentially misleading advice after their battles with the disease.

High-profile patients help raise awareness about diseases, and the news that two federal politicians were diagnosed with prostate cancer raised hopes of more funding for research to find a cure.

However, doctors are angry about the promotion of a diagnostic tool called the prostate specific antigen (PSA) test, a blood test many experts find pointless, unproven and unreliable.

Mr. Manning, the Alliance MP and former Reform Party leader, credited it with saving his life. The doctor for Mr. Rock, the Health Minister, advises men to get regular PSA tests.

However, many doctors say there is no proof that regular PSA tests for men with no symptoms cut the prostate cancer death rate. Patients are now asking for the test, often paying out of their own pocket, despite their doctors' misgivings.

"Does the fact that Mr. Rock was diagnosed with prostate cancer change the evidence on PSA screening?" asked Dr. Michelle Greiver, a Toronto family physician. "It does not. Yet, we find that our patients are being told to ask us to order this test for them, because of the publicity surrounding the minister's disease."

Doctors also worry men who find prostate examinations an uncomfortable experience will opt for the PSA test alone, which can fail to detect up to 20% of prostate cancers. That decision could cost lives, they say.

Screening tests are supposed to catch cancers early and allow patients to live longer. This is true for colorectal and breast cancer screening.

Yet while early detection means men like Mr. Rock and Mr. Manning can have prostate surgery, studies suggest the benefits of this treatment have been overestimated.

"Does early detection by prostate screening reduce death?" asked Dr. Richard Schabas, head of Cancer Care Ontario, a government agency.

"We don't know. So I get upset, I must say, when I hear people -- including some doctors -- who only present one side of the evidence, and who don't make it clear that there's an absolutely critical piece of evidence that's missing."

The controversy has already made Mr. Manning cautious.

Despite his promise to speak out about prostate cancer, the Calgary MP declined to grant the National Post an interview when told it would include questions about PSA screening.

Mr. Rock's family history - his father died of prostate cancer - put him in a high-risk group for the disease, which may warrant a regular PSA screening test. However, the majority of men do not fit into this category.

Discovered over 20 years ago, PSA is a protein produced by the prostate. A high level of that protein may indicate the presence of cancerous or pre-cancerous cells.

However, it can also lead to unnecessary worry.

According to the Canadian Institute for Clinical Evaluative Sciences, 10% of men over 50 years of age who are tested will have elevated levels of PSA. Less than a third of those will be diagnosed with cancer after further tests; the rest will undergo the needless stress and discomfort of further tests, possibly including a biopsy.

More worrisome, 2% of those tested will have prostate cancer that is not detected by the PSA. For this reason, doctors regard a physical exam as a key diagnostic test.

"The problem is PSA is not specific for prostate cancer," said Dr. Mario D'Costa, a clinical biochemist and director of the core laboratory at St. Joseph's Health Centre in Toronto.

"It is elevated also in benign disease and also in acute inflammation of the prostate."

Many patients are treated aggressively for prostate cancer based on a high PSA result, including surgery, radiation and hormone therapy. In some cases this can lead to impotence, incontinence and other problems.

"These are the risks associated with treating a disease known in the majority of patients to be slow-growing - quite often, men outlive the disease and die of other things."

Dr. D'Costa's lab, one of the first in Canada to process PSA tests, handles about 160,000 tests a year, much of it screening of healthy patients.

Several years ago, when prostate cancer patient General Norman Schwarzkopf appeared on the CNN show Larry King Live to talk about PSA, the demand for tests rose sharply. Dr. D'Costa expects the publicity surrounding Mr. Rock and Mr. Manning to cause a similar jump.

"With the awareness that's occurred, I'm sure that the vast majority of people want to have this test because they think it's the magic test that's going to tell them whether they've got cancer or not," he said.

But some experts, including urologists who specialize in prostate care, say concerns about the PSA test are overblown.

They point out the death rate due to prostate cancer in Canada declined by 9.6% between 1991 and 1996. In the U.S., the rate has fallen 1% a year since 1990. This trend has coincided with the rise in PSA screening.

While some data suggest that had been due to better treatments, including hormone therapy, radiation and prostate surgery, others say the PSA test has already proved its value.

"We understand why family physicians said five or six years ago: 'Wait a minute, are we diagnosing too many patients?' " asked Dr. Mostafa Elhilali, chairman of the urology department at McGill University.

"Are we treating too many patients who don't need to be treated? In other words, would they have died of something else and not of prostate cancer? Are we doing a lot of extra testing, biopsies and things like that, putting patients through a lot of pain, for nothing? These are questions on everybody's mind."

But Dr. Ehlilali says time has proved the answer to these questions is generally: "No."

He said any doctor who fails to offer a PSA test is depriving a patient of an important tool.

"If you do rectal exams but say no to a PSA test, then you are missing 80% of the prostate cancers," he said. "If you really want to find prostate cancers, then you better use the PSA."