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Prostate-specific antigen (PSA) is a widely used biochemical marker for prostate cancer (1). PSA, a serine protease, forms complexes with serine protease inhibitors such as α2-macroglobulin and α1-antichymotrypsin (ACT) (2). The major immunoreactive forms of serum PSA are PSA-ACT complex and free PSA. PSA complexed to α2-macroglobulin is not measured by current immunoassays (3). Because the proportion of PSA complexed to ACT is higher in men with prostate cancer than in men with benign prostatic hyperplasia (BPH) (3)(4), the use of a ratio of free-to-total PSA may improve discrimination between men with BPH and prostate cancer (5)(6).Results obtained with different PSA immunoassays are not interchangeable (7)(8)(9)(10). The variation among immunoassays reflects, in large part, the specificity of different PSA antibodies (7). To a lesser extent, the composition of the calibrator, the PSA values assigned to the calibrator, the test diluent, and the physical design of the immunoassays contribute to intermethod differences (6)(7). Assays using a monoclonal-monoclonal sandwich format yield, in general, an equimolar response to free and ACT-complexed PSA, whereas monoclonal-polyclonal formats tend to yield higher values for samples containing greater proportions of free PSA (8). We have recently described the Bayer Immuno 1TM PSA Assay (Bayer Corporation), which uses a monoclonal-polyclonal antibody format that yields an equimolar response to varying proportions of free and complexed PSA (11). This has now been found to be a result of the unique binding properties of the monoclonal antibody used for the capture of PSA (manuscript in submission).Proficiency testing among clinical laboratories has indicated large intermethod variability in the quantitation of total PSA (12). The aim of this study was to determine the magnitude and cause of this intermethod variation. Here we report our multi-site analysis of …

Variation in the Quantitation of Prostate-specific Antigen in Reference Material: Differences in Commercial Immunoassays,