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Prostate-specific antigen: biochemistry, analytical methods, and clinical application.

Prostate-specific antigen (PSA) is a glycoprotein produced exclusively by prostatic tissue. PSA's absolute tissue specificity makes it valuable as a forensic marker and, more important, as a tumor marker for prostatic cancer. Prostatic cancer is prevalent in the older male population and is a major cause of death in men. Previously, prostatic acid phosphatase (PAP) was used to help diagnose and monitor the efficacy of therapy for prostate cancer. PAP has now been displaced by PSA, which has greater clinical sensitivity even though it has less clinical specificity. PSA is useful for monitoring therapy, particularly surgical prostatectomy, because complete removal of the prostate gland should result in PSA being undetectable. Measurable PSA after radical prostatectomy indicates residual prostatic tissue or metastasis, and increasing PSA concentrations indicate recurrent disease. PSA is also useful for screening selected populations of patients with symptoms indicative of prostate cancer; its use for general screening is debatable because of its less-than-optimal specificity, the cost of unselected screening, and the lack of evidence that early detection of prostate cancer decreases morbidity and mortality. Distinguishing between patients with prostatic cancer and those with benign prostatic hypertrophy is particularly difficult because of the overlap in PSA values in the two groups. Determining the rate of change in PSA per year from serial measurements or calculating the ratio of PSA per volume of the prostate gland may allow these two groups to be more readily differentiated.