title: Reproducibility and accuracy of measurements of free and total prostate-specific antigen in serum vs plasma after long-term storage at -20 degrees C.
abstract: BACKGROUND
  Long-term frozen storage may alter the results of prostate-specific antigen (PSA) measurements, mainly because of degradation of free PSA (fPSA) in vitro. We compared the effects of long-term storage on fPSA, total PSA (tPSA), and complexed PSA (cPSA) in serum vs EDTA-plasma samples.
  
  
  METHODS
  We measured fPSA and tPSA concentrations in matched pairs of archival serum and EDTA-plasma samples (stored frozen at -20 degrees C for 20 years) from a large population-based cohort in Malmö, Sweden. We also compared concentrations in age-matched men with those in samples not subjected to long-term storage, obtained from participants in a population-based study of prostate cancer screening in Göteborg, Sweden. These contemporary samples were handled according to standardized preanalytical and analytical protocols aimed at minimizing in vitro degradation. tPSA and fPSA measurements were performed with a commercial assay (Prostatus Dual Assay; Perkin-Elmer Life Sciences).
  
  
  RESULTS
  Concentrations of tPSA and fPSA and calculated cPSA (tPSA - fPSA) in archival plasma were not significantly different from those in contemporary serum from age-matched men. In archival serum, however, random variability of fPSA was higher vs plasma than in contemporary samples, whereas systematic error of fPSA analyses was similarly small in archival and contemporary serum and plasma.
  
  
  CONCLUSIONS
  Concentrations of tPSA and calculated cPSA were highly stable in plasma and serum samples subjected to long-term storage at -20 degrees C. Greater random variability, rather than a systematic decrease, may explain differences in fPSA analyses observed in archival serum.
authors.affiliation: Department of Laboratory Medicine, Division of Clinical Chemistry, Lund University, University Hospital (UMAS), Malmö, Sweden. David.Ulmert@med.lu.se
authors.forename: David
authors.initials: D
authors.lastname: Ulmert
authors.forename: Charlotte
authors.initials: C
authors.lastname: Becker
authors.forename: Jan-Ake
authors.initials: JA
authors.lastname: Nilsson
authors.forename: Timo
authors.initials: T
authors.lastname: Piironen
authors.forename: Thomas
authors.initials: T
authors.lastname: Björk
authors.forename: Jonas
authors.initials: J
authors.lastname: Hugosson
authors.forename: Göran
authors.initials: G
authors.lastname: Berglund
authors.forename: Hans
authors.initials: H
authors.lastname: Lilja
mesh_terms: D000368:Aged; D015331:Cohort Studies; D015925:Cryopreservation; D006801:Humans; D008297:Male; D008875:Middle Aged; D010949:Plasma / Q000737:chemistry*; D017430:Prostate-Specific Antigen / Q000097:blood*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D044967:Serum / Q000737:chemistry*; D013048:Specimen Handling; D013548:Sweden; D013997:Time Factors
chemical_list: D017430:Prostate-Specific Antigen