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Dual-Label Immunoassay for Simultaneous Measurement of Prostate-specific Antigen (PSA)-₁-Antichymotrypsin Complex Together with Free or Total PSA

¹ Department of Clinical Chemistry, Helsinki University Central Hospital, Biomedicum, PB 700, FIN-00029 Helsinki, Finland.

^aAuthor for correspondence. Fax 358-9-47171737; e-mail ulf-hakan.stenman{at}hus.fi.

Background: A major portion of prostate-specific antigen exists in circulation as a complex with ₁-antichymotrypsin (PSA-ACT), whereas a minor part is free (fPSA). The proportion of PSA-ACT is increased in prostate cancer (PCa), but immunologic determination of PSA-ACT is hampered by a background produced by nonspecific adsorption of ACT to the solid phase. To reduce the nonspecific interference, we produced an antibody specific for complexed ACT and developed immunofluorometric assays (IFMAs) for simultaneous measurement of fPSA + PSA-ACT (fPSA/PSA-ACT) and PSA-ACT + total PSA (tPSA, PSA-ACT/tPSA).

Methods: Monoclonal antibodies (MAbs) were produced by immunization with PSA-ACT. The dual-label time-resolved IFMAs for fPSA/PSA-ACT and PSA-ACT/tPSA used a capture MAb to tPSA, an Eu³⁺-labeled MAb to fPSA or complexed ACT, and an Sm³⁺-labeled MAb to complexed ACT or to tPSA as tracer antibodies. The clinical utility was evaluated using serum samples from individuals with or without PCa with PSA concentrations of 2.0–20.0 µg/L.

Results: One MAb (1D10) showed low cross-reactivity with free ACT and cathepsin G-ACT. A sandwich assay for PSA-ACT with 1D10 as tracer had a detection limit of 0.05 µg/L, and with this assay, PSA-ACT was undetectable in female sera. The detection limit for fPSA was 0.004 µg/L. Determinations of the ratio of fPSA to PSA-ACT and the proportions of fPSA/tPSA and PSA-ACT/tPSA provided the same clinical specificity for PCa and provided significantly better clinical specificity than did tPSA.

Conclusions: Background problems observed in earlier PSA-ACT assays are eliminated by the use of a MAb specific for complexed ACT as a tracer. The same clinical validity can be obtained by determination of fPSA or PSA-ACT together or in combination with tPSA.

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