PSA (Prostate-specific antigen)

Prostate-specific antigen (PSA) is a serine protease of the kallikrein family produced almost exclusively by prostatic epithelial cells; it liquefies seminal coagulum but leaks into the bloodstream in proportion to the degree of disruption of prostatic glandular architecture. Elevated serum PSA (commonly flagged above 4 ng/mL, though thresholds vary by age and risk profile) can reflect prostate cancer, benign prostatic hyperplasia, prostatitis, or recent mechanical stimulation, and has a positive predictive value for cancer of roughly 20–30% at levels of 4–10 ng/mL, though this estimate is substantially modified by age, race, DRE findings, and PSA density. Derived metrics — PSA density, PSA velocity, free-to-total PSA ratio, and age-specific ranges — improve specificity and reduce unnecessary biopsy. PSA-based screening has shown a modest reduction in prostate cancer mortality in some randomised trials (notably ERSPC) but not others (PLCO), and detects considerable indolent disease, leading to shared decision-making recommendations (USPSTF Grade C for ages 55–69) about its use as a longevity screening tool.