Prostate specific antigen (PSA) is a 33-kDa glycoprotein secreted by epithelial cells of the prostate gland. In human serum, PSA is primarily complexed with α1-antichymotrypsin, and to a lesser extent with other serum proteins. Only a small portion of PSA is present as the free form (free PSA). The expected normal level of total PSA in male serum is lower than 4 ng/mL. A rise in the concentration of PSA indicates prostate pathology, including benign prostatic hyperplasia and prostate cancer.