Prostate-specific antigen (PSA) is a substance released into the blood by the prostate, which is part of the male reproductive system. 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 people. The amount of PSA in the blood normally increases as the prostate enlarges with age. It is also increased by ejaculation, by inflammation of the prostate gland (prostatitis), and by prostate cancer.