Comparison of specific interferon gamma response in blood and tuberculin skin test for the diagnosis of tuberculosis infection in tuberculosis contacts

The tuberculin skin test used to detect latent Mycobacterium tuberculosis infectionhas many drawbacks, and a new diagnostic test for latent tuberculosis (QuantiFERON-TBGold assay) has recently been introduced. This test measures the production of interferongamma in whole blood upon stimulation with ESAT-6 and CFP-10 which are specificantigens for M. tuberculosis. The aim of the study is to compare the tuberculin skin testand the new method, whole blood IFN-gamma assay in the tuberculosis contacts for thediagnosis of latent tuberculosis infection. The assays were evaluated in 233 individualsfrom three risk groups: Group 1, 133 household contacts of smear positive index cases;Group 2, 46 tuberculosis contacts of the community of smear positive index cases; andGroup 3, 74 health care workers. The rates of positive results of the tuberculin skin test andIFN-gamma assay in the cases were 37% and 42%, respectively. There was no significantdifference in results of the tuberculin skin test among the three patient groups. But thepositive result (51.3%, p=0.013) of the IFN-gamma assay in Group 1 was higher than theother groups. A poor agreement between the two tests was found in our risk groups(65.7%, kappa value: 0.28). Agreement between the tests was moderate in unvaccinatedpeople (72.7%, kappa value: 0.44). Negative agreement was higher than the positiveagreement (44.0% versus 28.7%) in unvaccinated people. Our results demonstrate that theIFN-gamma assay based on ESAT-6 and CFP-antigens has limited value for diagnosis oflatent tuberculosis infection in our country where BCG vaccination programme is routine.Further studies could address improvement of the reliability of new tests with M.tuberculosis specific antigens.Key words: Mycobacterium tuberculosis, infection, diagnosis, interferon gamma