Variation of free and total prostate-specific antigen levels: the effect on the percent free/total prostate-specific antigen
Nixon, R.G.; Lilly, J.D.; Liedtke, R.J.; Batjer, J.D.
Archives of Pathology and Laboratory Medicine 121(4): 385-391
1997
ISSN/ISBN: 0003-9985 PMID: 9140308 Document Number: 471912
Objective: Recent studies have shown that calculations of the percent free/total prostate-specific antigen (PSA) improves the specificity of PSA testing. Characterizing the variability of free PSA and total PSA is necessary to evaluate the utility of an isolated free/total PSA measurement. We investigated the total variation of free and total PSA levels to determine how the percent free/total PSA was affected. Design: Serum was obtained from nine urological patients on 5 different days over a 2-week period. Free and total PSA levels were measured on the day of collection. The total variation expressed in terms of percent coefficient of variation (%CV) was calculated, and the biological variation was derived taking analytical variation into consideration. Setting: Patients were from Seattle (Wash) Urological Associates, and samples were processed at the Dynacare Laboratory of Pathology, Seattle, Wash. Patients: Nine men (aged 48 to 69 years) were evaluated; three had been diagnosed with prostate cancer, three with benign prostatic hyperplasia, one with chronic prostatitis, one with high-grade prostatic intraepithelial neoplasia, and one was clinically normal. Main Outcome Measures: Total variation for free, total, and percent free/total PSA. Results: The average total variation was 13.9% CV, 7.5% CV, and 10.6% CV for free, total, and percent free/total PSA, respectively. Biological variation was derived to be 13.0% CV, 5.6% CV, and 8.0% CV for free, total, and percent free/total PSA, respectively. Conclusions: When applied, these results suggest that there are significant random changes in the numerator and denominator of the free PSA-total PSA ratio that could result in clinical misinterpretation. Clinicians must be aware that free PSA and total PSA levels will fluctuate owing to nonpathologic variation.