title: Reproducibility of interferon gamma (IFN-γ) release Assays. A systematic review.
abstract: BACKGROUND
  Interferon gamma (IFN-γ) release assays for latent tuberculosis infection result in a larger-than-expected number of conversions and reversions in occupational screening programs, and reproducibility of test results is a concern.
  
  
  OBJECTIVE
  Knowledge of the relative contribution and extent of the individual sources of variability (immunological, preanalytical, or analytical) could help optimize testing protocols.
  
  
  METHODS
  We performed a systematic review of studies published by October 2013 on all potential sources of variability of commercial IFN-γ release assays (QuantiFERON-TB Gold In-Tube and T-SPOT.TB). The included studies assessed test variability under identical conditions and under different conditions (the latter both overall and stratified by individual sources of variability). Linear mixed effects models were used to estimate within-subject SD.
  
  
  RESULTS
  We identified a total of 26 articles, including 7 studies analyzing variability under the same conditions, 10 studies analyzing variability with repeat testing over time under different conditions, and 19 studies reporting individual sources of variability. Most data were on QuantiFERON (only three studies on T-SPOT.TB). A considerable number of conversions and reversions were seen around the manufacturer-recommended cut-point. The estimated range of variability of IFN-γ response in QuantiFERON under identical conditions was ±0.47 IU/ml (coefficient of variation, 13%) and ±0.26 IU/ml (30%) for individuals with an initial IFN-γ response in the borderline range (0.25-0.80 IU/ml). The estimated range of variability in noncontrolled settings was substantially larger (±1.4 IU/ml; 60%). Blood volume inoculated into QuantiFERON tubes and preanalytic delay were identified as key sources of variability.
  
  
  CONCLUSIONS
  This systematic review shows substantial variability with repeat IFN-γ release assays testing even under identical conditions, suggesting that reversions and conversions around the existing cut-point should be interpreted with caution.
authors.affiliation: 1 Division of Respirology.
authors.forename: Saloua
authors.initials: S
authors.lastname: Tagmouti
authors.forename: Madeline
authors.initials: M
authors.lastname: Slater
authors.forename: Andrea
authors.initials: A
authors.lastname: Benedetti
authors.forename: Sandra V
authors.initials: SV
authors.lastname: Kik
authors.forename: Niaz
authors.initials: N
authors.lastname: Banaei
authors.forename: Adithya
authors.initials: A
authors.lastname: Cattamanchi
authors.forename: John
authors.initials: J
authors.lastname: Metcalfe
authors.forename: David
authors.initials: D
authors.lastname: Dowdy
authors.forename: Richard
authors.initials: R
authors.lastname: van Zyl Smit
authors.forename: Nandini
authors.initials: N
authors.lastname: Dendukuri
authors.forename: Madhukar
authors.initials: M
authors.lastname: Pai
authors.forename: Claudia
authors.initials: C
authors.lastname: Denkinger
mesh_terms: D006801:Humans; D007371:Interferon-gamma / Q000097:blood* / Q000276:immunology; D015203:Reproducibility of Results; D014373:Tuberculin / Q000276:immunology; D014376:Tuberculosis / Q000097:blood / Q000175:diagnosis / Q000276:immunology
chemical_list: D014373:Tuberculin; D007371:Interferon-gamma
keywords: IFN-γ release assays; diagnostics; reproducibility; tuberculosis