Performance of confirmatory interferon-γ release assays in school TB outbreaks

Interferon-γ release assays (IGRAs) have been incorporated into several national guidelines for latent TB infection (LTBI) diagnosis. However, their optimal application is still controversial and evolving. The aim of this study is to evaluate the performance of confirmatory IGRAs in addition to tuberculin skin tests (TSTs) in high school students with TST-positive (TST+) results who have had contact with another student who had TB (referred to in this article as "contacts") in TB outbreaks in a high BCG-vaccinated population.

We conducted a retrospective observational study of contacts in five school TB outbreaks in South Korea. The progression rates of TB within 2 years were compared among the groups based on the results of TSTs and QuantiFERON-TB gold assays (QFT-Gs).

Among 1,826 contacts, 21 (1.2%) developed active TB. Of the untreated groups, the rate of progression to TB was higher in the group with TST-positive (TST+) results (6.1%, six of 99) than in that with TST-negative (TST-) results (0.6%, 10 of 1,556; P < .001). Among TST+ contacts, the rate of progression to TB was higher in the group with QFT-G-positive (QFT-G+) results (18.75%, six of 32) than that with QFT-G-negative (QFT-G(-)) results (0%, 0 of 67; P = .001). None of the 67 contacts with TST+/QFT-G(-) results progressed to active TB.

The addition of a confirmatory IGRA for TST+ contacts could effectively focus the targeting of LTBI treatment to fewer contacts in an intermediate-incidence setting in a high BCG-vaccinated population.