The proper interval for repeating an interferon-γ release assay (IGRA) among tuberculosis contacts with initially negative results is unknown.
We evaluated the interval for IGRA conversion after exposure to patients with active pulmonary tuberculosis in an outbreak setting. In a platoon of 32 soldiers, four active pulmonary tuberculosis patients, in addition to one index patient, were diagnosed during a contact investigation. For the other 27 contacts, a tuberculin skin test (TST) and QuantiFERON(®) TB Gold In-Tube assay (QFT-GIT) were performed. For soldiers with a negative result on the initial QFT-GIT, the test was repeated at 2, 4, 8, 14, 18, and 30 weeks, until positive conversion occurred. When conversion was identified, the subject was treated for latent tuberculosis infection. Initially, 17 (63.0%) soldiers had positive QFT-GIT results, whereas 21 (77.8%) showed positive TST results. Among 10 participants with initially negative QFT-GIT results, three had conversion at 2 weeks; three, at 4 weeks; and three, at 14 weeks.
Conversion did not occur in one contact during 30-week observation period. Based on the tuberculosis exposure time points among the contacts, IGRA conversion generally occurred 4-7 weeks after exposure, although it could occur as late as 14-22 weeks after exposure.
