Development of an immunoassay specific for the PSA-ACT complex without the problem of high background

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Abstract
We have developed an assay specific for the PSA‐ACT (PSA‐α1‐antichymotrypsin) complex that effectively diminishes the problem of high assay background commonly reported by other investigators. The assay follows a two‐site ELISA format. Polyclonal anti‐PSA antibodies were coated on the microplate to capture the PSA complex from the serum, whereas the biotinylated anti‐ACT polyclonal antibodies and HRP‐conjugated streptavidin were used for detection. The high background ordinarily associated with this assay was greatly reduced when milk casein was added in addition to albumin for blocking and when the Super Block™ was also included in the diluents for sample dilution and dilution of enzyme conjugated detecting antibodies. The assay has a sensitivity of 0.05 ng/mL. The within‐run precision ranges from 4.2–7.2% and the between‐run precision falls between 5.8–8.5%. Cross reactions with ACT and free PSA (fPSA) are 0.0001% and 0.02%, respectively. The highest concentration of PSA‐ACT complex in the maternal sera was < 0.4 ng/mL by this assay, much less than reported in the literature. Using this improved assay, the sum of fPSA and PSA‐ACT concentrations were less than that of their corresponding total PSA (tPSA) most of the time. We believe that this improved assay should be used to replace the current tPSA assay for screening, monitoring, and managing patients with prostate cancer. J. Clin. Lab. Anal. 12:14–19, 1998. © 1998. Wiley‐Liss, Inc.