title: Interchangeability of measurements of total and free prostate-specific antigen in serum with 5 frequently used assay combinations: an update.
abstract: BACKGROUND
  The comparability of total and free prostate-specific antigen (tPSA and fPSA) results among commercial PSA assays has been suggested to be improved by calibration to WHO PSA reference materials and the development of equimolar-response assays. To characterize the current situation, we assessed 5 frequently used commercial assay combinations for tPSA and fPSA regarding the interchangeability of the PSA values and the ratio of fPSA to tPSA (%fPSA), equimolar characteristics, and diagnostic accuracy.
  
  
  METHODS
  Sera from 314 patients with prostate cancer (PCa) and 282 men with no evidence of prostate cancer (NPCa) were measured with tPSA and fPSA assays from Abbott (AxSYM), Beckman Coulter (Access), Diagnostic Products Corporation (Immulite 2000), and Roche (Elecsys 2010) and with tPSA and complexed PSA (cPSA) assays from Bayer (ADVIA Centaur).
  
  
  RESULTS
  Method comparisons (Passing and Bablok regressions; Bland-Altman plots) showed assay-dependent results for tPSA, fPSA, and %fPSA. With the Access tPSA values taken as 100%, tPSA concentrations varied from 87% (AxSYM and ADVIA Centaur) to 115% (Immulite), leading to different numbers of patients classified according to the commonly recommended tPSA cutoffs for performing a biopsy. Different %fPSA values also led to assay-dependent ROC analysis results, a finding that shows the importance for the diagnostic accuracy.
  
  
  CONCLUSIONS
  Interchangeability of tPSA, fPSA, and %fPSA values obtained by commercial PSA assays remains inadequate, but attention to this issue may minimize the misinterpretation of PSA results obtained by different assays.
authors.affiliation: Department of Urology, University Hospital Charité, Berlin, Germany.
authors.forename: Carsten
authors.initials: C
authors.lastname: Stephan
authors.forename: Moritz
authors.initials: M
authors.lastname: Klaas
authors.forename: Christian
authors.initials: C
authors.lastname: Müller
authors.forename: Dietmar
authors.initials: D
authors.lastname: Schnorr
authors.forename: Stefan A
authors.initials: SA
authors.lastname: Loening
authors.forename: Klaus
authors.initials: K
authors.lastname: Jung
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D006801:Humans; D007118:Immunoassay / Q000592:standards; D008297:Male; D008875:Middle Aged; D017430:Prostate-Specific Antigen / Q000097:blood* / Q000378:metabolism / Q000592:standards; D011471:Prostatic Neoplasms / Q000175:diagnosis; D011485:Protein Binding; D012372:ROC Curve; D012015:Reference Standards; D012016:Reference Values; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D044967:Serum
chemical_list: D017430:Prostate-Specific Antigen