title: Validation of 5 routine assays for serum free testosterone with a candidate reference measurement procedure based on ultrafiltration and isotope dilution-gas chromatography-mass spectrometry.
abstract: METHODS
  The analytical validity of free testosterone (FTe) analog immunoassays is subject to much controversy. We revisited the validation of 4 analog assays and 1 FTe calculation procedure with a metrologically traceable reference measurement procedure (RMP) based on ultrafiltration and isotope dilution-mass spectrometry for direct measurement of Te in the ultrafiltrate. To this end, we performed split-sample measurements of 40 male sera.
  
  
  RESULTS
  Deming regression showed that 3 of the immunoassays had moderate to good correlation (0.8474 < or = r < or = 0.9241) with the RMP; however, the slope was markedly below 1. The FTe calculation procedure was in good agreement with this result. The Sy/x values for all assays were higher than the combined imprecision values, which indicate their susceptibility to matrix-related effects.
  
  
  CONCLUSIONS
  The study demonstrated substantial differences in analytical quality of FTe assays; however, the results suggested that after extending the validation with a larger variety of samples, recalibration of some analog assays might be worth further investigation.
authors.affiliation: Laboratory for Analytical Chemistry, Faculty of Pharmaceutical Sciences, Ghent University, Harelbekestraat 72, B-9000 Gent, Belgium.
authors.forename: K
authors.initials: K
authors.lastname: Van Uytfanghe
authors.forename: D
authors.initials: D
authors.lastname: Stöckl
authors.forename: J M
authors.initials: JM
authors.lastname: Kaufman
authors.forename: T
authors.initials: T
authors.lastname: Fiers
authors.forename: A
authors.initials: A
authors.lastname: De Leenheer
authors.forename: L M
authors.initials: LM
authors.lastname: Thienpont
mesh_terms: D000328:Adult; D002138:Calibration; D002247:Carbon Isotopes; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D007006:Hypogonadism / Q000175:diagnosis; D007118:Immunoassay / Q000379:methods; D007201:Indicator Dilution Techniques; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013739:Testosterone / Q000097:blood*; D014462:Ultrafiltration
chemical_list: D002247:Carbon Isotopes; D013739:Testosterone