PSA is a substance produced by the prostate. It is mostly found in semen. The levels of PSA in the blood may be higher in men who have prostate cancer or other conditions. A PSA test is used primarily to screen for prostate cancer. A PSA test measures the amount of prostate-specific antigen (PSA) in your blood. Small amounts of PSA ordinarily circulate in the blood. The PSA test may detect high levels of PSA that could indicate the presence of prostate cancer. However, many other conditions, such as an enlarged or inflamed prostate, may also increase PSA levels.