PSA blood test vs prostate MRI

A PSA test is a blood sample that measures prostate-specific antigen, a protein produced by prostate cells. A higher level may reflect cancer, but also benign enlargement, inflammation or infection. Because it is a number rather than a picture, it cannot say where a problem is.

A prostate MRI uses magnets and radio waves to look at the gland in detail. It can show areas that are suspicious for significant cancer, assign a score for how likely this is, and guide a targeted biopsy. It cannot detect everything, and some low-grade cancers are invisible on MRI.

PSA can be normal in someone with prostate cancer and raised in someone without it. Age matters, since normal levels rise with age, and the threshold for concern is age-specific. A single measurement may be repeated after a few weeks if there is a reason for a temporary rise.

This is why most guidelines advise a discussion about benefits and harms before testing: finding slow-growing cancers that might never have caused problems can lead to treatment with side effects, a recognised issue called overdiagnosis.

MRI reduces unnecessary biopsies by identifying who has no suspicious area, and improves detection of significant cancers by targeting the sample. But it requires access to a scanner and a radiologist experienced in prostate reporting, and the quality of the study matters. A result is graded on a standard scale, and borderline grades often prompt further discussion.

It also does not replace a biopsy where a suspicious area is found. A biopsy gives the grade of the cancer, which guides treatment choices.

NICE guidance on prostate cancer diagnosis and management (NG131) recommends offering multiparametric MRI as the first-line investigation for people with suspected localised prostate cancer, with biopsy decisions guided by the result. This is the reason a raised PSA is commonly followed by MRI before biopsy.

The sequence reflects the aim of finding clinically important cancers while avoiding unnecessary biopsies, which carry risks of infection and bleeding.

• No symptoms and considering a check: discuss the pros and cons of a PSA test with your GP rather than going straight to scanning.

• Urinary symptoms such as frequent night-time urination: PSA and examination are the starting point, since many causes are benign.

• PSA above the age-specific threshold: expect referral and, in most pathways, an MRI before any biopsy.

• Strong family history or known gene variant: ask a specialist about earlier or more frequent testing.

• Cannot have an MRI: alternatives such as a different biopsy approach can be discussed with a urologist.

A raised PSA does not mean cancer. Many raised levels are caused by benign changes. And a normal PSA does not guarantee no cancer, though it makes significant cancer less likely.

Equally, MRI is not a screening tool for men with no symptoms and a normal PSA. Used that way it can find incidental changes of uncertain meaning.