Interferon-γ (IFN-γ) release assays (IGRAs), which can overcome several limitations of the tuberculin skin test in detecting latent TB infection, were introduced recently in routine clinical practice. However, IGRAs are highly dynamic tests, and the T-cell responses tend to fluctuate over time. We wanted to evaluate the conversion and reversion rates of monthly IGRAs over 1 year among health-care workers (HCWs) in contact with patients with TB in South Korea, a country with an intermediate TB burden.
Forty-nine HCWs in contact with patients with active pulmonary TB were prospectively enrolled. Interviews on exposure to patients with active TB, a physical examination, and a QuantiFERON TB Gold In-Tube assay (QFT-GIT) were carried out monthly for 1 year.
Among 48 participants, 25 (52%) showed inconsistent results in serial testing. When an increase from< 0.35 IU/mL to ≥ 0.70 IU/mL was applied as the definition of a conversion, inconsistent IGRA results were identified in 13 participants (27.0%). Consistency in QFT-GIT results was associated with age, length of service, and baseline IFN-γ levels, but not with the profession of the participant, degree of TB exposure, or wearing of an N95 mask. In five participants, fluctuations in IFN-γ levels showed levels > 0.70 IU/mL two or more times.
Fluctuation in IGRA findings was common when HCWs in contact with patients with TB were tested monthly. This could be a result of poor reproducibility of the assay, repeated infection and true reversion, or periodic secretion of antigens from Mycobacterium tuberculosis.
ClinicalTrials.gov; No.: NCT01121068; URL: www.clinicaltrials.gov