Comparison of Hybritech and WHO standardization

Background. It is widely accepted that the introduction of the First International Standard for PSA assay (WHO96/670) improved the agreement between total PSA immunoassay. Less attention was given to the fact that the results obtained with WHO calibration turned out to be lower by 16-20% than the original Hybritech assay with the inevitable consequences on positive/negative false results. The aim of the study was to verify, on UniCel®DxI800, the misalignment between Hybritech and WHO calibration for total PSA assay and to calculate the new cut-off value if WHO calibrated. Methods. We measured total PSA values, in 500 samples, both with Hybritech and WHO calibration on UniCel®DxI800 (Beckman Coulter). Results. Our data confirm the negative bias of WHO values with respect to Hybritech values and on the grounds of Passing-Bablok regression, the 4.0 μg/L cut-off, obtained according to Hybritech total PSA assay, corresponds to a WHOcalibrated cut-off of 3.2 μg/L. Conclusions. When WHO calibration is used for a PSA assay, a new assay-dependant cut-off should be calculated or, if the laboratory decides to maintain the same cut-off value of PSA-Hyb with a WHO calibration, a redefinition of the sensibility/specificity of 4.0 μg/L cut-off is needed.