Ultrasensitive time-resolved immunofluorometric assay of prostate-specific antigen in serum and preliminary clinical studies

Abstract

We developed an ultrasensitive method for measuring prostate-specific antigen (PSA) in serum. The assay includes a capture monoclonal anti-PSA antibody coated to microtiter wells, a biotinylated rabbit polyclonal detection antibody, and alkaline phosphatase (ALP)-labeled streptavidin. The activity of ALP is measured with the substrate diflunisal phosphate; the released diflunisal forms highly fluorescent complexes with Tb3+-EDTA that are quantified with microsecond time-resolved fluorometry. The assay is precise and accurate and correlates well with the established Hybritech Tandem™-PSA kit. Its distinguishing feature is extreme sensitivity (lowest limit of detection is 0.002 μg/L or 2 × 106 PSA molecules per assay). This is the most sensitive PSA assay reported thus far; we used it to quantify PSA in patients who had undergone radical prostatectomy. Many patients had <0.01 μg/L PSA in their serum. This method could have important clinical applications in postsurgical early detection of relapse or residual prostate cancer, as recently suggested in the literature (Clin Chem 1992;38:1930-2).