PSA (Prostate-specific antigen)

Prostate-specific antigen (PSA) is an enzyme (a kallikrein-family serine protease) made almost only by prostate cells. Its normal job is to liquefy semen, but it leaks into your blood in proportion to how disrupted the prostate's structure is. A high serum PSA (often flagged above 4 ng/mL, though thresholds vary by age and risk) can reflect prostate cancer, an enlarged prostate (BPH), prostatitis, or recent mechanical stimulation. At levels of 4 to 10 ng/mL, its positive predictive value for cancer is only about 20 to 30%, and even that shifts with age, race, exam findings, and PSA density. Derived metrics sharpen it: PSA density, PSA velocity, the free-to-total PSA ratio, and age-specific ranges. They improve specificity and cut unnecessary biopsies. As a screen, PSA reduced prostate-cancer death modestly in some randomized trials (like ERSPC) but not others (PLCO). It also detects a lot of harmless, slow disease. That is why guidelines call for shared decision-making (the USPSTF gives it a Grade C for ages 55 to 69).

Last reviewed: June 22, 2026

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