A single-chain, 31-kD glycoprotein with 240 amino acid residues and 4 carbohydrate side-chains; a kallikrein protease produced by prostatic epithelial cells and normally found in seminal fluid and circulating blood. Elevations of serum PSA are highly organ-specific but occur in both cancer (adenocarcinoma) and benign disease (e.g., benign prostatic hyperplasia, prostatitis). A significant number of patients with organ-confined cancer have normal PSA values. See: carcinoma of the prostate. Syn: human glandular kallikrein 3.
Levels of PSA less than 4 ng/mL (4 mg/L) are considered normal; levels exceeding 10 ng/mL (10 mg/L) are strongly indicative of prostatic carcinoma. Approximately 30% of patients with PSA levels between those limits will have prostate cancer detectable by biopsy within 1 year. Measurement of both free PSA and PSA that is complexed with the protease inhibitor )-1-antichymotrypsin (PSA-ACT) enhance the sensitivity of testing for carcinoma in men with total PSA levels between 4 and 10 ng/dL. The percentage of free PSA is lower in the serum of men with prostate cancer than in patients with normal prostates or benign disease. However, total PSA is a more accurate predictor of prostate cancer than the free-to-total PSA ratio. A level of free PSA that is 25% or more of total PSA in a patient with a palpably benign gland effectively rules out the need for prostatic biopsy when total PSA is less than 10 ng/mL. A level of free PSA of 15% or less strongly suggests carcinoma. An annual increase in PSA of more than 0.75 ng/mL [0.75 mcg/L] is also highly suggestive of malignancy. The PSA level may be elevated by prostatitis, recent ejaculation, and prostatic massage, but not by digital prostate examination; it may be depressed by therapy with finasteride or saw palmetto. Some studies have shown that PSA correlates with total prostate volume, which may become a confounding factor in benign prostatic hyperplasia. One large study showed that as many as one third of elevated PSA levels returned to normal spontaneously on later testing. The use of PSA tesing as well as other diagnostic maneuvers to screen asymptomatic old men for prostate cancer is controversial. Neither the U.S. Preventive Services Task Force nor the Natioinal Cancer Institute recommends PSA testing for routine screening. Even those authorities who recommend screening after age 50 (age 40–45 in African-American men and those with a family history of prostate cancer) do not advise screening in men with life expectancy of less than 10 years, because the 10-year survival rate of prostatic carcinoma is about 90%.
Reference: Stedman's Medical Dictionary