Prostate-specific antigen (PSA) is a substance released into a man's blood by his prostate gland. A PSA test measures the amount of prostate-specific antigen in the blood.

Low amounts of PSA may be found in the blood of healthy men. The amount of PSA in the blood normally increases as a man's prostate enlarges with age. It is also increased by inflammation of the prostate gland (prostatitis) and by prostate cancer.