PURPOSE Interferon-gamma release assay (IGRA) results have been suggested as a surrogate marker of treatment response in latent tuberculosis infection (LTBI). However, data have not been consistent, and most previous studies focused on participants taking isoniazid prophylaxis. The aim of this study was to elucidate the changes in the IGRA results in patients who underwent chemoprophylaxis with isoniazid and rifampicin daily for 3 months.

METHODS In a TB outbreak cohort, 26 asymptomatic close contacts with normal chest radiographs and positive QuantiFERON-TB Gold In-Tube assay (QFT-GIT) results were recruited. These patients were treated with isoniazid and rifampicin daily for 3 months. The QFT-GIT was repeated at 3 and 6 months following treatment initiation.

RESULTS Compared with the initial QFT-GIT results (3.59 ± 3.39 IU/mL), the interferon-gamma (IFN-γ) levels had decreased significantly at 6 months (0.84 ± 1.14 IU/mL; P = 0.005), but not at 3 months (3.58 ± 3.64 IU/mL; P = 0.98). Reversions occurred in seven (26.9 %) patients at 3 months and in an additional two participants at 6 months; a total of nine participants (34.6 %) had reversions. Recent conversion was associated with reversion of the test results (odds ratio 26.3, 95 % confidence interval 3.04-226.6).

CONCLUSION Chemoprophylaxis with isoniazid and rifampicin generally decreased IFN-γ levels among tuberculosis contacts. However, only a small portion of participants achieved reversion.