The potential for blood tests that detect cancer is huge.

But what if a test is, in some ways, too good? What if it finds cancers that if left undetected wouldn’t cause a person any harm? And what if it’s not possible to know for certain which cancers need to be treated, or who could avoid a life-altering diagnosis and potential long-term treatment side effects?

This is the situation right now with prostate cancer.

There’s no UK screening programme for prostate cancer. That’s because the blood test that would be used – called the PSA (or prostate specific antigen) test – isn’t reliable enough.

Men over the age of 50 with no symptoms of prostate cancer can still ask for the test in the UK. But its use is hotly debated around the world.

Now, a new Cancer Research UK-funded study, from scientists at the Universities of Bristol and Oxford, sheds more light on how unreliable the test is. And it confirms that for men without symptoms, the PSA test doesn’t save lives.

What is the PSA test?

Prostate cancer is the most common cancer in men, with 46,690 cases diagnosed each year in the UK.

The PSA blood test is one of the main ways, along with a rectal exam, that doctors can look for signs of prostate cancer.

The test measures the level of PSA in a blood sample. And while it’s normal for men to have some PSA in their blood, a raised level can be a sign of prostate cancer.

But PSA can be raised for lots of other reasons too, meaning a man might get an abnormal result when he doesn’t have cancer.

PSA test prostate cancer screening

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It’s this, combined with the test missing some cancers and finding others that would never cause a man any harm, that makes it unreliable for screening.

What does the new study show?

The CAP trial is the largest to date looking at whether the PSA test can reduce the number of men dying from prostate cancer. It includes over 400,000 men from almost 600 GP practices in the UK. Some practices invited men aged 50 to 69 for a one-off appointment with a nurse where they were offered information about PSA testing and, if they wanted it, the test. The rest operated as normal, and didn’t invite men for the test.

After an average of 10 years, the researchers found that the one-off PSA test led to many more prostate cancers being found. But crucially, the men who had the test were no less likely to die of prostate cancer than the men that hadn’t had it.

This suggests that giving a one-off PSA test to men without symptoms doesn’t save lives from prostate cancer, because it:

  • Picks up cancers that are unlikely to cause a person any harm
  • Misses cancers that are aggressive and probably would benefit from treatment

Professor Richard Martin, who led the study from the University of Bristol, says the results show that giving a one-off PSA test to men without symptoms “risks diagnosing more men with a cancer that would never have caused them any harm”.

“In some cases, this might mean men unnecessarily living with the stigma of having a cancer and the side effects of treatment that was not needed, such as incontinence and erectile dysfunction, for many years, maybe even decades,” he adds.

This backs up a study from 2013 that pooled data from 5 of the largest and most rigorous trials on prostate cancer screening. It too found that the test leads to many men being diagnosed with a cancer that did not need to be found. And that screening for prostate cancer using the PSA test doesn’t reduce the number of men dying from the disease.

PSA prostate cancer screening data

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What’s the problem with finding more prostate cancers?

It seems counter-intuitive to suggest that some cancers would be better off left alone. Especially when early diagnosis can boost treatment success for some cancers. But these overdiagnosed cancers pose a serious problem.

What is overdiagnosis?

Some cancers grow fast and spread quickly, but some grow so slowly that if they went undetected they wouldn’t cause a person any problems. People with these harmless cancers won’t have any symptoms, and they won’t die from the disease.

In other words, it doesn’t matter if the cancer is never found.

When these slow-growing cancers are found they’re said to be overdiagnosed.

Read more: Overdiagnosis – when finding cancer can do more harm than good

Unfortunately, there’s no way of telling apart the cancers that need to be found and treated and the ones that could have been left alone. This means that some patients will receive an unnecessary diagnosis, unnecessary treatment, and the emotional and physical side effects they come with.

Richard Roope, a GP and one of Cancer Research UK’s senior clinical advisors, says this highlights the challenge of diagnosing prostate cancer early.

“Some areas of medicine are very clear cut, others less so,” he says. “PSA testing falls very much into the latter. Prostate cancer comes in many shapes and sizes, ranging from the aggressive to the very slow growing. Unfortunately, no available test will discriminate between these.

“What we know is that giving men without symptoms a PSA test does not change the ultimate outcome if cancer is present, meanwhile any treatment used may produce unpleasant side effects.”

What should you do if you’re worried about prostate cancer?

This study shows that having a PSA test when you don’t have any prostate cancer symptoms comes with some serious risks. If you’re still thinking about having the test, it’s important to get the full picture to help you decide. Our website and your doctor can help.

For those with symptoms, or if you notice anything that’s not normal for you, visit your doctor. In most cases it won’t be cancer, but it’s still best to get it checked.

What next for prostate cancer research?

Over 11,000 men die from prostate cancer in the UK every year. And Professor Martin says it’s here that research must be focussed.

We’re not saying that screening for prostate cancer should be discounted. What we’re saying is PSA testing for prostate cancer is not the answer

– Professor Richard Martin

“Finding a way to detect and successfully treat these cancers, ones that are aggressive and harmful, is what we must continue to strive for.

“We’re not saying that screening for prostate cancer should be discounted. What we’re saying is PSA testing for prostate cancer is not the answer.”

One of the most pressing challenges for researchers is finding a way to distinguish the potentially lethal cancers that need treating from the harmless ones that don’t. This would be an important first step in addressing the problem of overdiagnosis of prostate cancer.

One area that’s showing promise is the use of specialised scans. A type of MRI has shown potential in clinical trials as a way of reducing the number of men who need to go through uncomfortable prostate biopsies. The scans could also help accurately map the size and location of prostate tumours and to identify those cancers that are more likely to be aggressive and need treatment.

Our goal is to save men from harm. This means finding a way to stop men with aggressive prostate cancer dying from their disease by developing better tests and more effective treatments. But as the latest research shows, this also means sparing men with harmless cancers an unnecessary but life-altering diagnosis.

Katie Edmunds is a health information officer at Cancer Research UK

Our research on prostate cancer

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Cancer Research UK-funded research Early detection Prostate cancer Research and trials Screening Tests