Discrepancy between the tuberculin skin test and the whole-blood interferon γ assay for the diagnosis of latent tuberculosis infection in an intermediate tuberculosis-burden country

Context. A recently developed whole-blood interferon γ (IFN-γ) assay based on stimulation with the Mycobacterium tuberculosis-specific antigens early secreted antigenic target 6 and culture filtrate protein 10 shows promise for the diagnosis of latent tuberculosis (TB) infection. Objective. To compare the tuberculin skin test (TST) and the whole-blood IFN-γ assay in the diagnosis of latent TB infection according to the intensity of exposure. Design and Setting. A prospective comparison between the whole-blood IFN-γ assay and the TST using a 2-TU dose of purified protein derivative RT23 in a population with intermediate TB burden was conducted sequentially between February 1, 2004, and February 28, 2005, in a Korean tertiary referral hospital. Participants. Of 273 participants, 220 (95.7%) had received BCG vaccine. Participants were grouped according to their risk of infection: group 1, no identifiable risk of M tuberculosis infection (n = 99); group 2, recent casual contacts (n = 72); group 3, recent close contacts (n = 48); group 4, bacteriologically or pathologically confirmed TB patients (n = 54). Main Outcome Measures. Levels of agreement between the TST and the IFN-γ assay and the likelihood of infection in the various groups. Results For the TST with a 10-mm induration cutoff, the positive response rate in group 1 was 51%; group 2, 60%; group 3, 71%, and group 4, 78%. For the IFN-γ assay, the positive response rate in group 1 was 4%; group 2, 10%; group 3, 44%; and group 4, 81%. The overall agreement between the TST and the IFN-γ assay in healthy volunteers was κ = 0.16. The odds of a positive test result per unit increase in exposure across the 4 groups increased by a factor of 5.31 (95% confidence interval [CI], 3.62-7.79) for the IFN-γ assay and by a factor of 1.52 (95% CI, 1.20-1.91) for the TST (P<.001). Using a 15-mm induration cutoff for the TST did not make a substantial difference to the test results. Conclusion. The IFN-γ assay is a better indicator of the risk of M tuberculosis infection than TST in a BCG-vaccinated population.