Comparison of a polyclonal and monoclonal immunoassay for PSA: need for an international antigen standard

Two leading commercial immunoassays for prostate specific antigen (the Yang Laboratories polyclonal radioimmunoassay and the Hybritech two-site monoclonal radioimmunometric assay) were compared using independently purified antigen as the calibrator. The Yang polyclonal assay yielded values from 1.4 to 1.9 times higher than the Hybritech monoclonal assay on the same sample, similar to results of previous investigators. When an independent prostate specific antigen calibrator was substituted for the respective kit calibrators, both assays yielded essentially identical results, thereby confirming that differences in assay values arise from differences in the assigned values of the kit calibrators. Otherwise, both assays had similar responses in the high and low range. Recovery studies of prostate specific antigen in various diluents demonstrated that antigen recovery was lowest in diluents with the highest protein content. We conclude that the kit manufacturers have assigned different values to their antigen calibrators. Sample values between the assays can be compared by use of an appropriate conversion table. Differences in their recommended ranges of normal for prostate specific antigen in male serum (zero to 2.5 ng./ml. for the Yang assay and zero to 4.0 ng./ml. for the Hybritech assay) reflect two factors: a difference in assigned calibrator values and differences in the selection of the "normal" population of men used to define the normal range of values. We urge adoption of an international standard calibrator for prostate specific antigen as a means to correct these differences.