Evaluation of an interferon-gamma release assay in young contacts of active tuberculosis cases.

In a cross-sectional study in a hospital in Tehran in 2006-08 the QuantiFERON-TB interferon-gamma release assay (QTB) was compared with the tuberculin skin test (TST) in 59 young people (aged < 20 years) with close contact with immunocompetent cases of proven pulmonary tuberculosis. After 1 year follow-up 10 subjects had progressed to tuberculosis disease and received treatment; TSTwas positive in 30% and QTB in 100%. Of the 49 non-progressive subjects, TST was positive in 10.4% and QTB in 16.3%. The agreement between TST and QTB assay in non-progressive subjects was poor (K = 0.43). False positive and false negative rates for TST were 40.0% and 9.3% respectively; positive and predictive values were 60.0% and 90.7%. We suggest adding the interferon assay to the skin test in the decision to perform chest X-ray or to start chemoprophylaxis at least in younger subjects (aged < 20 years).