False positive results - besides being elevated by prostate cancer, PSA levels can be raised due to urinary tract infections or even certain activities such as recent ejaculation and some forms of exercise. This means there is a risk of PSA being elevated when no cancer is present (a false positive). For every 100 men with a high PSA result, approximately 75 will have a false positive, and 25 will have a true positive (i.e. they will have prostate cancer). Currently, the most reliable way of working out whether you have a true positive or a false positive is through a prostate biopsy. As the PSA test has a high false positive rate, many prostate biopsies will prove unnecessary. However, MRI scans before a biopsy mean that unnecessary biopsies are in decline.
False negatives - this is where the PSA result is in the normal range, but the person has underlying prostate cancer. PSA levels can remain normal in the early stages of prostate cancer, providing false reassurance. If we test 100 men with prostate cancer, approximately 15 would have a normal PSA result (a false negative).
In some cases, prostate cancer grows very slowly and may never cause symptoms or shorten lifespan. A significant proportion of the men who receive a true positive result may undergo surgery they did not need.