title: Within-subject variability and boosting of T-cell interferon-gamma responses after tuberculin skin testing.
abstract: BACKGROUND
  The optimal strategy for the diagnosis of latent tuberculosis infection is controversial. Adoption of a two-step strategy (tuberculin skin test [TST] followed by an IFN-gamma release assay [IGRA], compared with an IGRA alone), may be limited by TST-mediated boosting of subsequent IGRA responses. Assessment of within-subject IGRA variability will aid in establishing thresholds for conversions and reversions, and interpretation of serial testing results.
  
  
  OBJECTIVE
  To determine short-term IGRA variability and the impact of TST on subsequent IGRA results.
  
  
  METHODS
  Within-subject variability and TST-mediated boosting of IGRA responses were evaluated in 26 South African participants with varying exposure risk. IGRAs (T-SPOT.TB, QuantiFERON-TB Gold In-Tube [QuantiFERON-TB-GIT], PPD, and heparin-binding hemagglutinin) were repeated four times over 21 days pre-TST, and on Days 3, 7, 28, and 84 post-TST administration.
  
  
  RESULTS
  All participants showed within-subject IGRA variability. Changes of +/-3 spots (T-SPOT.TB) or +/-80% from the mean IFN-gamma response (QuantiFERON-TB-GIT) over 3 weeks explained 95% of the variability. Spontaneous conversions/reversions occurred in 7 of 26 subjects (27%) (6 for T-SPOT.TB and 1 for QuantiFERON-TB-GIT [P = 0.049]) during the within-patient variability studies (pre-TST). After the TST eight subjects (33%) boosted above the defined baseline variability. By Day 7 post-TST, but not Day 3, 2 (12.5%) initially IGRA-negative test subjects converted. By contrast, boosting of PPD and heparin-binding hemagglutinin occurred by Day 3 post-TST.
  
  
  CONCLUSIONS
  When using a two-step screening strategy it appears safe to perform a QuantiFERON-TB-GIT or T-SPOT.TB IGRA within 3 days of performing the TST. A 3-spot or 80% IFN-gamma response variation, on either side of baseline values, explains 95% of the short-term variability and may be useful for interpreting conversions and reversions, and values close to the cut-point.
authors.affiliation: Lung Infection and Immunity Unit, Division of Pulmonology and UCT Lung Institute Department of Medicine, University of Cape Town, Cape Town, South Africa.
authors.forename: Richard N
authors.initials: RN
authors.lastname: van Zyl-Smit
authors.forename: Madhukar
authors.initials: M
authors.lastname: Pai
authors.forename: Kwaku
authors.initials: K
authors.lastname: Peprah
authors.forename: Richard
authors.initials: R
authors.lastname: Meldau
authors.forename: Jackie
authors.initials: J
authors.lastname: Kieck
authors.forename: June
authors.initials: J
authors.lastname: Juritz
authors.forename: Motasim
authors.initials: M
authors.lastname: Badri
authors.forename: Alimuddin
authors.initials: A
authors.lastname: Zumla
authors.forename: Leonardo A
authors.initials: LA
authors.lastname: Sechi
authors.forename: Eric D
authors.initials: ED
authors.lastname: Bateman
authors.forename: Keertan
authors.initials: K
authors.lastname: Dheda