COMPARISON OF 3 INVESTIGATIONAL ASSAYS FOR THE FREE-FORM OF PROSTATE-SPECIFIC ANTIGEN

Purpose: Measuring percent free/total prostate specific antigen (PSA) versus total PSA alone has been shown to enhance the stratification of cases with and without prostate cancer. We compared the diagnostic pe rformance of 3 free PSA assays when values were interchanged over a co mmon total PSA denominator. Materials and Methods: Archival sera from 123 consecutively accrued patients (25 with prostate cancer, 98 with n o evidence of malignancy) who had pre-biopsy total PSA levels from 2.0 to 20.0 ng./ml, were obtained from our specimen bank. Serum specimens were analyzed using the Hybritech Tandem R,dagger Dianon Systems doub le dagger and Chiron ACS 180<SUP></SUP> free PSA assays. Total PSA was measured using the Hybritech Tandem R total PSA assay. Linear regress ion was performed to evaluate the correlation and agreement among assa ys. Sensitivity and specificity of cancer detection were calculated to compare diagnostic performance. Results: The required percent free PS A cutoffs were different among the 3 free PSA assays. The cutoffs to e nsure a 95% sensitivity of cancer detection were 22, 34 and 34%, and t he number of negative biopsies that were prevented at these cutoffs (s pecificity) was 38, 19 and 33% for the Hybritech, Dianon, and Chiron a ssays, respectively. Conclusions: The results among the different assa ys were not interchangeable, Clinicians should be aware that different percent free PSA cutoffs need to be used based on the type of free an d total PSA assays, and that not all assays diagnostically perform the same.