Interferon-gamma release assays during follow-up of tuberculin skin test-positive contacts.

Setting: Following a large-scale contact investigation, individuals with a positive tuberculin skin test (TST) result were offered preventive tuberculosis treatment.
Objective: To investigate the effect of isoniazid (INH) treatment and the effect of time on interferon gamma release assay (IGRA) results during follow-up.
Design: TST-positive subjects (n = 122) detected during the large-scale contact investigation were included in the study. Blood was obtained every 6 months over 2 years to perform both tests.
Results: Preventive INH treatment was completed by 36 of the 122 (29.5%) subjects, 71 (58.2%) were followed up with 6-monthly X-ray screening and 15 (12.3%) did not complete INH treatment. The overall percentage of individuals with a positive result remained stable during the 2 years, at approximately 45-50%, but individual responses varied over time. The majority of initially low IGRA results remained below the cut-off value, initially high IGRA results remained positive, while initially intermediate IGRA results were followed by more dynamic patterns.
Conclusion: This study showed a highly variable pattern of IGRA responses over time and suggests limited value for their use during follow-up of latently infected individuals. However, the significance of different kinetic patterns observed among subjects with intermediate initial IGRA results warrants further study.