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

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