Young Ae Kang, MD; Hye Won Lee, MSc; Ho Il Yoon, MD; BeLong Cho, MD; Sung Koo Han, MD; Young-Soo Shim, MD; Jae-Joon Yim, MD
Context A recently developed whole-blood interferon (IFN-) 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- 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- 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- 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- 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- assay in healthy volunteers was = 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- 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- assay is a better indicator of the risk of M tuberculosis infection than TST in a BCG-vaccinated population.
Author Affiliations: Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine and Lung Institute (Drs Kang, Yoon, Han, Shim, and Yim and Ms Lee) and Department of Family Medicine (Dr Cho), Seoul National University College of Medicine, Seoul, Republic of Korea.