The total prostate-specific antigen (PSA) serum test measures the total concentration of PSA protein in the blood. PSA is an enzyme primarily produced by cells in the prostate gland. Its physiological function is to liquefy seminal fluid. PSA enters the bloodstream in small amounts when the blood-prostate barrier is disrupted, either due to abnormal growth (hyperplasia, inflammation, cancer) or medical procedures. It is used as an important tumor marker, but it is not specific for cancer alone.