Interchangeability and diagnostic accuracy of two assays for total and free prostate-specific antigen: two not always related items.

The variation between different PSA assays seems to influence the interpretation of individual PSA values and the clinical decisions about prostate cancer. One reason for this variability could be the different reactivity of antibodies for the various molecular forms of serum PSA; as a result, samples containing the same amount of tPSA but different proportions of fPSA can produce very different values. In this study, serum samples were collected prospectively from 152 consecutive patients referred to 2 institutions (Regional Hospital, Venice, 90 subjects; San Bortolo Hospital, Vicenza, 62 subjects) for PSA elevation and/or symptoms. Serum samples were assessed according to the manufacturers' instructions on the following 2 analyzers: the Immulite 2000 assay (Diagnostic Products Corporation, Los Angeles, USA), which measures tPSA and fPSA, and the ADVIA Centaur (Bayer Diagnostics, Tarrytown, USA), which assays tPSA and cPSA. cPSA values were transformed into fPSA by the equation fPSA=tPSA-cPSA. When taking Immulite tPSA and f/tPSA values as 100%, ADVIA Centaur values were 92.6% and 122%, respectively, which means that 20% of patients would be classified differently according to the traditional biopsy cutoff. In conclusion, there are considerable differences between the 2 methods, which could affect clinical decisions.