BACKGROUND: The metrological traceability of prostate-specific antigen (PSA) assay calibration to WHO standards is desirable to potentially improve the comparability between PSA assays. A method comparison was performed between the traditionally standardized Beckman Coulter Hybritech Access PSA and free PSA (fPSA) assays and a new alternate calibration of assays aligned to the WHO standards 96/670 and 96/668, respectively. METHODS: Sera from 641 men with and without prostate cancer, various control materials and mixtures of different proportions of the WHO standards were measured with both assay calibrations. RESULTS: Excellent comparability between the corresponding assay calibrations was observed, with correlation coefficients of at least 0.996. The Passing-Bablok slopes were 0.747 for total PSA (tPSA), 0.776 for fPSA and 1.02 for the percentage ratio of fPSA to tPSA (%fPSA), while the corresponding percentages of the new WHO-aligned assay results related to the traditional assays were 76.2%, 77% and 102.2%. Receiver operating characteristics revealed no differences between the two PSA assay calibrations. CONCLUSIONS: The WHO calibration yields results approximately 25% lower for tPSA and fPSA values when compared with the conventional Hybritech calibration. Using the WHO-aligned PSA assay, a tPSA cut-off of 3 microg/L should be considered in clinical practice, while %fPSA cut-offs could be retained.
BACKGROUND: The metrological traceability of prostate-specific antigen (PSA) assay calibration to WHO standards is desirable to potentially improve the comparability between PSA assays. A method comparison was performed between the traditionally standardized Beckman Coulter Hybritech Access PSA and free PSA (fPSA) assays and a new alternate calibration of assays aligned to the WHO standards 96/670 and 96/668, respectively. METHODS: Sera from 641 men with and without prostate cancer, various control materials and mixtures of different proportions of the WHO standards were measured with both assay calibrations. RESULTS: Excellent comparability between the corresponding assay calibrations was observed, with correlation coefficients of at least 0.996. The Passing-Bablok slopes were 0.747 for total PSA (tPSA), 0.776 for fPSA and 1.02 for the percentage ratio of fPSA to tPSA (%fPSA), while the corresponding percentages of the new WHO-aligned assay results related to the traditional assays were 76.2%, 77% and 102.2%. Receiver operating characteristics revealed no differences between the two PSA assay calibrations. CONCLUSIONS: The WHO calibration yields results approximately 25% lower for tPSA and fPSA values when compared with the conventional Hybritech calibration. Using the WHO-aligned PSA assay, a tPSA cut-off of 3 microg/L should be considered in clinical practice, while %fPSA cut-offs could be retained.