prostate-specific antigen

Prostate-specific antigen, or PSA, is a protein produced by the cells of the prostate gland, by normal cells as well as cancerous ones. The PSA test measures how much of it is circulating in the blood. The US National Cancer Institute describes three uses: monitoring men already diagnosed with prostate cancer, investigating prostate symptoms, and screening men who have no symptoms at all.

A raised level is not a diagnosis. The institute says there is no single threshold that separates a normal result from an abnormal one, although a level above 4.0 nanograms per millilitre is generally treated as abnormal and may lead to a recommendation for a biopsy. Benign enlargement of the prostate, inflammation, infection, a recent biopsy, vigorous exercise and ejaculation can all push the figure up for a time, so a doctor will often repeat the test after six to eight weeks before going further.

That is why PSA screening is weighed as carefully as it is. About 6 to 7 percent of men screened in a given round get a false-positive result, and some of the cancers the test does find are tumours that would never have caused problems in the person's lifetime. Finding a cancer earlier does not always mean curing it, and treating one that did not need treating exposes a man to side effects such as urinary leakage, bowel problems and sexual dysfunction.