Discrepancy between the tuberculin skin test and the levels of serum interferon-gamma in the diagnosis of tubercular infection in contacts

ObjectiveOur aim was to compare the tuberculin skin test (TST) results and the level of serum IFN-γ in the diagnosis of TB infection among contact of smear positive tuberculosis.Design and methodsChest x ray, tuberculin skin test and serum level of interferon-gamma (IFN-γ) by ELISA were performed to 30 sputum positive tuberculosis patients, their 118 household contacts and 31 healthy controls.ResultsThe serum level of IFN-γ was significantly elevated in index cases than in contacts and control groups. There was no statically significant difference in serum level of IFN-γ between vaccinated and unvaccinated contacts. There was no significant correlation between IFN-γ level and tuberculin reaction or induration diameter in vaccinated contacts. There was significant correlation between IFN-γ level and tuberculin reaction or induration diameter in BCG unvaccinated contacts.ConclusionThe serum IFN-γ is a better indicator of the risk of mycobacterial infection than TST in BCG-vaccinated contacts.