Within-Subject Variability and Boosting of T-Cell Interferon-γ Responses after Tuberculin Skin Testing

Rationale. 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. Objectives: 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], PIPD, and heparin-binding hemagglutinin) were repeated four times over 21 days pre-TST, and on Days 3, 7, 28, and 84 post-TST administration. Measurements and Main 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 2 subjects (27%) (6 for T-SPOT.TB and I 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 shortterm variability and may be useful for interpreting conversions and reversions, and values close to the cut-point.