Association of the level of IFN-γ produced by T cells in response to Mycobacterium tuberculosis-specific antigens with the size of skin test indurations among individuals with latent tuberculosis in a highly tuberculosis-endemic setting

There is growing evidence showing the potential of T-cell-based gamma interferon (IFN-γ) release assays (IGRAs) for predicting the risk of progression of Mycobacterium tuberculosis (Mtb) infection, though there is little information from tuberculosis (TB)-endemic settings. In this study, we assessed the association between the level of IFN-γ produced by T cells in response to Mtb-specific antigens and the size of skin test indurations in 505 adult individuals who were screened for latent tuberculosis infection (LTBI) using the QuantiFERON-TB Gold In Tube (QFTGIT) assay and tuberculin skin test (TST). There was a strong positive correlation between the level of IFN-γ induced by the specific antigens and the diameter of the skin indurations (Spearman's rho 5 0.6, P < 0.001). Body mass index and parasitic infection were not associated with the level of IFN-γ production or the TST reaction. In linear regression analysis, the size of the skin test indurations was significantly associated with the mean level of IFN-γ [coefficient, 0.65; 95% confidence interval (CI), 0.47 to 0.82, P < 0.001]. Similarly, results from logistic regression analysis demonstrated that individuals who had skin test indurations ‡ 10 mm were 6.82 times more likely than individuals who had skin test indurations < 10 mm to have high levels of IFN-γ (i.e. positive QFTGIT result) (adjusted odd ratio 5 6.82; 95% CI, 3.67 to 12.69, P < 0.001). In conclusion, the results of this study could provide indirect evidence for the prognostic use of the QFTGIT assay for progression of Mtb infection, though prospective follow-up studies are needed to provide direct evidence.