Prostate Specific Antigen (PSA), a glycoprotein of the glandular kallikrein family, is a serine protease with chymotrypsin-like enzymatic activity. It has a molecular weight of approximately 33,000-34,000 Da and is found primarily in prostate tissue and seminal fluid. Serum levels of PSA are very low in healthy individuals (0-4 ng/ml), but are often elevated in malignant and benign prostatic disease. As such, immunoassays for serum levels of PSA have proven useful in the diagnosis and follow-up of prostate cancer.

PSA released into circulation is present as unbound, free PSA (fPSA) or bound to serum protease inhibitors, such as α1-Antichymotrypsin (ACT) and α2-Macroglobulin (A2M). Research indicates that immunoassays for serum levels of PSA alone cannot be relied on to distinguish prostate cancer from benign prostatic hyperplasia (BPH). Serum levels of PSA-ACT complex are reportedly higher in patients with prostate cancer than in those with BPH. As such, measuring the ratio of PSA-ACT to fPSA in patient samples is proving valuable in the differential diagnosis of prostate cancer. In addition, immunoassays for PSA-A2M serum levels may prove valuable in prostate cancer diagnosis, but further study is required.

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***Read how the addition of information from biological factors significantly increases the clinical utility of the serum PSA test:Scripps News Vol. 16 No. 2