Evaluation of the Abbott lMx#{174} Automated Immunoassay of Prostate-Specific Antigen

We detail the performance characteristics of the new IMx PSA immunoassay developed by Abbott Laboratories, addressing PSA recovery, assay reproducibility, standard curve storage, lower limit of detection, dilution linearity, and correlation with the Hybritech Tandem-A#{174} PSA immunoassay. We analyzed 686 sera for PSA retrospectively, testing 555 of these concurrently with the lMx and the Tandem-A immunoassays. The IMx PSA standard curve was linear from 0 to 100 .&g/L, and curve storage was maintained for 4 weeks. The lower limit of detection of the lMx PSA assay was 0.03 pg/L; allowing for the assay precision yielded a biological detection limit of 0.06 g/L. We conservatively set the clinical threshold at 0.1 pg/L. Regression analysis of dilution linearity involving 10 samples (0.44-200 pgIL) resulted in coefficients of correlation ranging from 0.9972 to 1.0000. Reproducibility studies with 18 specimens within the range of 0.3941 3.67 p.g/L gave intraand interassay CVs <6.5%. The interassay 95% confidence interval for a specimen containing 0.06 g of PSA per liter was 0.03-0.09 g/L. Correlation between IMx and Tandem-A PSA assay results was excellent: r = 0.9909 and slope = 0.95. Overall, the lMx PSA immunoassay, with the conveniences of automation, curve storage, and nonisotopic handling, provided an improved lower limit of PSA detection, which allows for earlier indication of residual or recurrent disease after radical prostatectomy.