Use of the prostate-specific antigen blood test to screen for prostate cancer in elderly men has been controversial. A panel of experts called the US Preventive Services Task Force last year urged doctors to stop screening men 75 and older.

The new findings involved 849 US men taking part in a study of aging run by the National Institutes of Health.

For men over 75, not one died of prostate cancer if they had a PSA reading below 3 nanograms per millilitre. That represents at least two thirds of the population.

"For the overwhelming majority of men over age 75, discontinuing PSA screening is probably a very safe thing to do," Dr H Ballentine Carter of Johns Hopkins University in Baltimore, one of the researchers in the study appearing in the Journal of Urology, said in a telephone interview.

But for men with a PSA higher of 3ng/ml, Carter said continued screening might be of value.

"What we found was that at any age, men with a PSA greater than 3, their risk of prostate cancer, and even an aggressive prostate cancer, during the remaining years of their life continues to increase," Carter said.

PSA is a protein produced by the prostate and its levels may shoot up when prostate tumours grow or spread. Higher PSA levels also can indicate a benign prostate enlargement.

Doctors consider PSA test results in deciding whether to order a biopsy to diagnose prostate cancer.

Many prostate tumours are slow-growing and may take years if they ever are to become dangerous to a man. But other tumours can be aggressive and life-threatening.

Because PSA screening has led to a big increase in prostate cancer diagnoses, some experts believe many men with tumours that pose little threat are getting treatment such as surgery and radiation that may be unnecessary.

Most of the men in the study lived in the Baltimore and Washington region; 122 had prostate cancer and 727 did not.