Abstract
The measurement of PSA is recommended for men over 50 years of age for screening of prostate cancer. However, proper differentiation of prostate cancer from benign prostate hyperplasia (BPH) relies on an accurate measurement of free PSA (fPSA) and a correct calculation of percent fPSA. Because of the extremely low concentration of fPSA in the serum, any slight deviation from its true value may produce large errors in percent fPSA calculated. Therefore, we undertook a study examining carefully those parameters of the fPSA assay which might affect the fPSA determination. We found that the integrity of the calibrator, the computer curve‐fitting program selected, the source of the calibrator, and the total PSA or fPSA + PSA complexes (tPSA) concentration of the specimen all had an impact on the accuracy of the fPSA value assayed. We found that an examination of the slope of the calibration curve was important to reveal whether the calibrator had or had not been denatured during storage. We also found that the 4‐parameter cure fitting program was best suited for plotting the fPSA calibration curve. The calibrator we isolated from LNCaP cells was acceptable for our assay because it had an affinity for the assay antibody very similar to that of serum fPSA. We also determined the effect of tPSA concentration on the fPSA determinations and found that within the concentration range of 4–10 ng/mL the impact on the percent fPSA calculated was not significant. We believe that our assay produces accurate fPSA values when all these assay parameters are well controlled. J. Clin. Lab. Anal. 12:304–309, 1998. © 1998 Wiley‐Liss, Inc.
Keywords: prostate cancer, screening, 4 parameter, benign prostate hyperplasia
References
- 1. Wu JT: Assay for prostate specific antigen (PSA): Current problems and possible solutions. J Clin Lab Anal 8: 51–62, 1994. [DOI] [PubMed] [Google Scholar]
- 2. Wu JT: Serum prostate‐specific antigen assay — An update. West J Med 162: 447–448, 1995. [PMC free article] [PubMed] [Google Scholar]
- 3. Lilja H, Christensson A, Dahlen U, et al.: Prostate‐specific antigen in serum occurs predominantly in complex with α1‐Antichymotrypsin. Clin Chem 37: 1618–1625, 1991. [PubMed] [Google Scholar]
- 4. Lilja H, Cockett ATK, Abrahamsson P‐A: Prostate‐specific antigen predominantly forms a complex with alpha1‐antichymotrypsin in blood. Cancer 70 (Suppl): 230–234, 1992. [DOI] [PubMed] [Google Scholar]
- 5. Wu JT, Zhang P, Wang T, Wilson L, Astill M: Evaluation of free PSA isoforms, PSA complex formation, and specificity of anti‐PSA antibodies by HPLC and PAGE‐immunoblotting techniques. J Clin Lab Anal 9: 1–14, 1995. [DOI] [PubMed] [Google Scholar]
- 6. Catalona W, Smith D, Ratliff T, et al.: Measurement of prostate‐specific antigen in serum as a screening test for prostate cancer. N Engl J Med 324: 1156–1161, 1991. [DOI] [PubMed] [Google Scholar]
- 7. Catalona WJ, Smith DS, Wolfert RL, et al.: Evaluation of percentage of free serum prostate‐specific antigen to improve specificity of prostate cancer screening. JAMA 274: 1214–1220, 1995. [PubMed] [Google Scholar]
- 8. Vashi AR, Oesterling JE: Percent free prostate‐specific antigen: Entering a new era in the detection of prostate cancer. Mayo Clin Proc 72: 337–344, 1997. [DOI] [PubMed] [Google Scholar]
- 9. Catalona WJ, Colberg JW, Smith DS, Ornstein DK, Shayka JJ: Measurement of percent free PSA improves specificity for lower PSA cutoffs in prostate cancer screening. J Urol 155 (Suppl): 422A, 1996. [Google Scholar]
- 10. Bangma CH, Rietbergen JEW, Kranse R, Blijenberg BG, Petterson K, Schroder FH: The free‐to‐total prostate specific antigen ratio improves the specificity of prostate specific antigen in screening for prostate cancer in the general population. J Urol 157: 2191–2196, 1997. [PubMed] [Google Scholar]
- 11. Wu JT, Liu GH, Zhan P, Stephenson RA: Monitoring percent free PSA in serial specimens: Improvement of test specificity, early detection and identification of occult tumors. J Clin Lab Anal 12: 26–31, 1998. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12. Wu JT, Stephenson RA: Microplate assay for free PSA, PSA‐ACT, and total PSA: Assay characteristics, selection of calibrators, and in vitro stability studies. Proceeding of the 1st International Consultation of Prostate Cancer. Monaco, 1997, p 159–168.
- 13. Peterman JH: Imunochemical considerations in the analysis of data from noncompetitive solid‐phase immunoassays In Immunochemistry of Solid‐Phase Immunoassay. Butler JE, ed. CRC Press, Boca Raton, Florida, 1991, p 47–65. [Google Scholar]
- 14. SOFTmax version 2.01 users manual. Molecular Devices Corporation, Menlo Park, California, 1989.
- 15. Daniels PB: The fitting, acceptance and processing of standard curve data in automated immunoassay systems, as exemplified by the Serono SRI analyzer. Clin Chem 40: 513–517, 1994. [PubMed] [Google Scholar]
- 16. Wu JT, Lyons BW, Liu GH, Wu LL: Production of milligram concentrations of free prostate specific antigen (fPSA) from LNCaP cell culture: Difference between fPSA from LNCaP cell and seminal plasma. J Clin Lab Anal 12: 6–13, 1998. [DOI] [PMC free article] [PubMed] [Google Scholar]