Enzyme Immunoassay for the Quantitative Determination of Free PSA Concentration in Human Serum
Introduction
Prostate-Specific Antigen (PSA) is a single-chain glycoprotein belonging to the kallikrein family, exhibiting enzymatic activity similar to chymotrypsin. The PSA glycoprotein monomer, with a molecular weight of approximately 30 kDa, is produced by the prostate epithelium and is primarily secreted into seminal fluid, where it facilitates the liquefaction of semen via proteolysis of gel-forming proteins (Belanger et al., 1995).
PSA exists in three primary forms in human serum. The first form is bound to alpha-2-macroglobulin, which renders it immunologically undetectable. The second form is a PSA complex bound to alpha-1-antichymotrypsin (ACT), with a molecular weight of 90-100 kDa, and remains detectable through immunoassays. The third form, known as free PSA, is not complexed with protease inhibitors and is also detectable by immunoassays (Zhov et al., 1993).
Total PSA (free PSA + PSA-ACT complex) levels are known to be elevated in benign prostatic hyperplasia (BPH) and prostate cancer. The ratio of free PSA to total PSA has been suggested as a diagnostic marker to improve differentiation between BPH and prostate cancer. Studies indicate that men with prostate cancer typically exhibit a lower free PSA/total PSA ratio than those with BPH when total PSA levels range from 4.0 to 10.0 ng/mL (McDemed, 2005). However, a definitive prostate cancer diagnosis requires a biopsy.
Clinical Significance
The assessment of free PSA levels in conjunction with total PSA measurements is clinically significant in distinguishing between benign and malignant prostate conditions. Elevated total PSA levels alone may be indicative of prostate disorders, but a lower percentage of free PSA (<10%) is associated with a higher likelihood of prostate cancer, whereas a higher percentage (>25%) suggests a lower probability (Belanger et al., 1995). This test is particularly useful in patients with total PSA levels between 4-10 ng/mL, aiding in risk stratification and clinical decision-making.
Test Principle
References
- Belanger, A., Van Harbeek, H., et al. (1995). Molecular mass and carbohydrate structure of prostate-specific antigen. Prostate, 27(3), 187-197.
- Zhov, A. M., Tewari, P. C., & Card, W. G. (1993). Multiple forms of PSA in serum: Differences in immunorecognition by monoclonal and polyclonal assay. Clinical Chemistry, 39, 2483.
- McDemed, J. (2005). Using PSA intelligently to manage prostate cancer. PCRI Insights, 8(3).