Standardization of PSA measures: a reappraisal and an experience with WHO calibration of Beckman Coulter Access Hybritech total and free PSA

Prostate-specific antigen (PSA) is the serum biomarker most widely used in prostate diseases. Since there is significant variation in PSA results among non-equimolar assays, the 90:10 ratio of complexed PSA to free PSA (the Stanford standard) was proposed as standardization; this became the basis for the PSA mass standards WHO 96/670 for tPSA and 96/668 for fPSA. Nevertheless, recent publications underlined the lack of interchangeability between different commercial assays, all claimed to be equimolar and calibrated to the WHO standard. Importantly, the WHO calibration yields about 16-20% lower PSA results. Manufacturers that have chosen to calibrate existing assays to the mass value of WHO 96/760 have introduced a significant negative bias compared to the Hybritech assay calibration; this bias is transferred to clinical evaluation if the cutoff of 4 ng/mL, clinically validated for the Hybritech assay, is maintained with the WHO calibration. Beckman Coulter recently provided the option of calibrating the Access Hybritech PSA and free PSA assays to the WHO standard introducing a different clinical cutoff. Using two different reagent lots, we tested about 200 routine patients for tPSA and fPSA with both calibrations; we also calculated the f/tPSA ratio with both calibrated methods. Moreover, we verified the analytical sensitivity and inter- and intra-assay variability. In accordance with the claim of the manufacturer, the results obtained with the WHO calibration showed a negative bias of about 25% and, as expected, no significant difference was found for % f/tPSA. The same bias was found when retesting samples of the External Quality Assessment Scheme of the Institute of Clinical Physiology of the National Research Council in Pisa. Based on this experience we decided for the moment to keep the Hybritech calibration, in order to avoid cutoff changes during patient follow-up. Moreover, we have started to provide information to clinicians aimed at the alignment of our results with the WHO standardization.