Multiple forms of prostate-specific antigen in serum: Differences in immunorecognition by monoclonal and polyclonal assays

Abstract

Prostate-specific antigen (PSA) in serum is primarily complexed with α1-antichymotrypsin (α1-ACT). However, 12-15% of prostate cancer (PCa) patients present with the predominant form being uncomplexed (free) PSA (Lilja et al., Clin Chem 1991;37:1618-24). We report that commercial immunoassays demonstrate variations in reactivity, especially to the uncomplexed form. We fractionated and analyzed commercial controls, PSA complexes prepared in vitro, and sera from patients with PCa or benign prostatic hyperplasia, using molecular sieve chromatography and Hybritech® Tandem®-R, Abbott IMx®, and Ciba Corning ACS™ PSA assays. Peak integration of PCa samples demonstrated ACS:Tandem-R ratios of 1-1.3 for PSA/α1-ACT complex. In contrast, ratios of uncomplexed peaks ranged from 2 to 4, suggesting a greater reactivity of the uncomplexed form in the ACS PSA assay. Discrepancies between assays, when PSA was measured in unfractionated sera, correlated directly with the percentage of the uncomplexed form. In controls, fractionation revealed the presence of uncomplexed PSA only, with ratios of ACS:Tandem-R and IMx:Tandem-R of 3:1 and 1.8:1, respectively, Immunoblots of PCa sera detected uncomplexed PSA (∼30 kDa) and PSA complexes of ∼95 kDa (PSA/α1-ACT) and >200 kDa, indicative of α2-macroglobulin. Maximal recognition of all forms of PSA may be important for early detection of disease progression.