On the standardization of total prostate-specific antigen:: an exercise with two reference preparations

In this study, 112 serum samples were analyzed for total prostate-specific antigen with three well-established assays i.e. Tandem R and Tandem E (both from Hybritech Inc., San Diego, USA) and Prostatus Free/Total from Wallac Oy, Turku, Finland. Thirty-two samples were collected from prostate cancer patients, 32 from patients with benign prostate hyperplasia and 48 from men participating in a screening study for prostate cancer. The aim of the study was to compare the results before and after recalculation with the data obtained with two reference preparations for total prostate-specific antigen: Stanford 90:10 PSA Calibrator and Certified Reference Material 613 Prostate-Specific Antigen. Comparing the actual results revealed almost perfect correlations between Tandem R and Tandem E and between both Tandem assays and Prostatus. We observed statistically significant differences in accuracy between Tandem R and Tandem E: y(Tandem E) = 1.05 x (Tandem R) + 0.07 and between Tandem E and Prostatus: y(Prostatus) = 0.94 x (Tandem E) + 0.02 In both comparisons prostate-specific antigen values ranged from 0-40 mu g/l. Recalculation with both reference preparations did not solve these discrepancies. One exception was the combination Tandem R and Tandem E. The application of either reference preparation solved the differences in accuracy here. In conclusion, even after recalibration, assays for total prostate-specific antigen are still not completely interchangeable.