Critical aspects related to the interpretation of the free-to-total PSA-ratio

Introduction: The number of assays available for the measurement of total a nd free PSA is increasing. As different methods can determine different PSA concentrations as well as different free-to-total PSA ratios in identical serum samples, the cut-offvalue for the ratio still needs to be determined. Methods: 114 sera from patients with histologically confirmed benign prost atic hyperplasia (BPH; n = 58) and cancer of the prostate (CaP; n = 56) wer e analyzed with two different assays. Free PSA (free), total PSA (total) an d the free-to-rotal- PSA ratio (ratio) were determined employing Enzym-Test PSA und freies PSA (Boehringer Mannheim, Germany) and Immulite PSA und fre ies PSA (DPC Biermann, Bad Nauheim, Germany) Results: The statistical resul ts are tabulated below: [GRAPHICS] Conclusion: Direct comparison of the two assays revealed a high statistical correlation (r = 0.94-0.99) for free and total PSA. In contrast, the ratio of the two assays was not as reproducible (r = 0.81-0.83). This result ind icates that the reference range for the ratio is dependent on the assay emp loyed and an that uncritical use of an applied reference range can be count er-productive.