20-25% lower concentrations of total and free prostate-specific antigen (PSA) after calibration of PSA assays to the WHO reference materials--analysis of 1098 patients in four centers.

AIM: To examine the potential clinical implications of the recalibration of total prostate-specific antigen (PSA) and free PSA (fPSA) assays to the World Health Organization (WHO) standard materials.
MATERIAL AND METHODS: Data from 1098 patients with or without clinically detected prostate cancer (PCa) from four independent cohort studies were compared using commercial assays calibrated to the traditional Hybritech PSA (PSA-Hyb) and fPSA (fPSA-Hyb) standards and to the WHO 96/670 (PSA-WHO) and 96/668 (fPSA-WHO) standards. The Access Immunoassay System (Beckman Coulter, Inc.) was used in all studies.
RESULTS: All studies showed 20% to 25% lower PSA and fPSA test results with the WHO-standardized assays. No significant change in %fPSA (fPSA/PSA x 100) was observed. Continuing to use the traditional clinical PSA cutoffs obtained with the Hybritech standard after changing to the PSA-WHO standard could result in up to one-third of prostate cancer cases being missed.
CONCLUSIONS: Manufacturers should fully inform laboratories about a calibration change and its clinical impact. Laboratory reports for PSA measurements should indicate the assay's manufacturer and which calibration standard was used to avoid misleading information concerning PCa risk.