PSA screening for prostate cancer has increased the number of diagnoses of the disease in Canada, but researchers at the University of Calgary say the test only minimally reduces the chances of dying from the disease.

According to a new study that will be published Tuesday in the online edition of CMAJ Open, it's likely that men who aren't screened using the PSA test will never even know they have cancer.

However, those who do develop cancer should be diagnosed and treated when symptoms appear, said the university's Dr. James Dickinson, the lead author of the paper. 

"The problem is that by screening a whole lot, we diagnose far more men than [those who] ever had prostate cancer," sad Dickinson. "That is, cancer in the sense that it would invade and spread and cause trouble and kill you."

Screening not worthwhile

In November, 2014, the Canadian Task Force on Preventative Health Care — of which Dickinson is a member — released a report recommending against PSA screening for prostate cancer.

Prostate cancer is a slow growing disease, the risk of which rises as men get older. Dickinson said treatment is not worthwhile for men who have slow-growing cancers that are not likely to invade and kill.

"People who supported this [the screening] argued that 'the mortality rate, the death rate is going down, so it must be working,'" Dickinson explained. "So we looked in detail at the death rate to try and understand what's really going on."

Canada has one of the highest rates of prostate cancer in the world, though mortality rates continue to drop by about 3.25 per cent annually. 

That's a decrease that cannot be attributed to screening alone, Dickinson said. The study's authors looked to other countries and determined that there are other factors to explain the decline, such as better treatment.

"We're diagnosing and treating a whole lot more men who were never going to die from the prostate cancer and therefore cannot benefit," said Dickinson. "All they get is the harms from the treatment."

Men who are uncertain are advised to discuss PSA screening with their family doctor "very carefully" before choosing to do so.