title: Clinical implications of introducing a new PSA assay.
abstract: BACKGROUND
  A number of different prostate-specific antigen (PSA) assays are in common use. There has been little consideration of the possible clinical implications of interassay variation. The availability of two assays in the same laboratory provided an opportunity to audit the clinical implications of the interassay variation in PSA levels.
  
  
  METHODS
  The same serum samples from patients with prostate cancer on follow-up were analyzed for PSA by the Abbott AxSYM assay and by the Abbott ARCHITECT assays. To assess within-patient reproducibility of the interassay variation, repeat analysis of PSA by both assays was conducted in a second sample obtained at least 1 month after the first.
  
  
  RESULTS
  Samples from 156 cases were analyzed. The mean ratio of serum PSA values by the two assays (AxSYM assay/ARCHITECT assay) was 0.89 (range 0.5-2.27). The interassay coefficient of variation was 20%. In a subgroup of 50 cases with repeat samples available, the correlation coefficient, r, of the interassay variation in PSA between the first and second samples was 0.441.
  
  
  CONCLUSIONS
  Interassay variation in serum PSA is clinically significant, both between patients and on repeated measurement within the same patient. Clinicians should be aware that simple correction factors may not accurately control for variation between PSA assays. Ideally, patients on follow-up for prostate cancer should be monitored using a single PSA assay.
authors.affiliation: Academic Unit of Radiotherapy & Oncology, Institute of Cancer Research, Downs Road, Sutton, Surrey SM2 5PT, UK.
authors.forename: Ramachandran
authors.initials: R
authors.lastname: Venkitaraman
authors.forename: Andrew R
authors.initials: AR
authors.lastname: Norman
authors.forename: Jhangir
authors.initials: J
authors.lastname: Iqbal
authors.forename: David P
authors.initials: DP
authors.lastname: Dearnaley
authors.forename: Alan
authors.initials: A
authors.lastname: Horwich
authors.forename: Robert A
authors.initials: RA
authors.lastname: Huddart
authors.forename: Chris C
authors.initials: CC
authors.lastname: Parker
mesh_terms: D014408:Biomarkers, Tumor / Q000032:analysis / Q000097:blood; D006801:Humans; D016014:Linear Models; D008297:Male; D011446:Prospective Studies; D017430:Prostate-Specific Antigen / Q000032:analysis / Q000097:blood*; D011471:Prostatic Neoplasms / Q000097:blood*; D015203:Reproducibility of Results
chemical_list: D014408:Biomarkers, Tumor; D017430:Prostate-Specific Antigen