Prostate specific antigen (PSA) is a serine protease with chymotrypsin-like activity. It is a member of the kallikrein-related peptidase gene family. PSA is a single-chain glycoprotein of 237 amino acids with a molecular weight of approximately 30,000 daltons. The major site of PSA production is the glandular epithelium of the prostate, but it has also been found in breast cancers, salivary gland neoplasms, breast milk, and other sources. PSA occurs in two major immunodetectable forms in blood. The major form is PSA complexed with the serine protease inhibitor, α-1-antichymotrypsin (PSA-ACT). Uncomplexed, or free, PSA is the other detectable form of PSA in serum. The Simoa™ Total PSA assay uses reagents that recognize both forms equally. Measurement of PSA following radical prostatectomy (RP) has become standard practice for prostate cancer recurrence monitoring. PSA is typically undetectable by conventional assay methods following surgery. However, low-abundance PSA could be rising while remaining undetected. Early adjuvant and salvage radiation therapies following surgery significantly improve patient outcomes, and recent clinical data with extreme sensitivity, non-conventional immunoassay (immunoPCR and digital immunoassay) indicate potential utility from low-abundance PSA measurement following RP for risk stratification and early cancer recurrence monitoring.