Prostate specific antigen (PSA) i s a 3 3-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 fraction of PSA is present as the free form (free PSA).
The expected normal level of PSA in male serum is lower than 4 ng/mL. A rise in the concentration of PSA indicates prostate pathology, including benign prostatic hyperplasia (BPH) and prostate cancer.
Free PSA (fPSA) has been studied in attempts to help distinguish BPH from untreated prostate cancer. These studies have shown that the ratio of free PSA / total PSA is lower in untreated prostate cancer than in patients with BPH.