Prostate specific antigen (PSA) is a protein produced primarily by cells in the prostate, a small gland that encircles the urethra in males and produces a fluid that makes up part of semen. Most of the PSA that the prostate produces is released into this fluid, but small amounts of it are also released into the bloodstream. This test measures the amount of PSA in the blood. Results that fall between 4.00 ug/L and 10.00 ug/L will automatically reflex to a Free PSA with a calculated ratio. The ratio of Free to Total PSA may help discriminate between prostate cancer and benign prostatic hyperplasia.