Following a large-scale contact investigation, individuals with a positive tuberculin skin test (TST) result were offered preventive tuberculosis treatment.To investigate the effect of isoniazid (INH) treatment and the effect of time on interferon gamma release assay (IGRA) results during follow-up.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.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.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.