Use of QuantiFERON-TB Gold test in the investigation of unexplained positive tuberculin skin tests

Summary Objectives To investigate the use of new gamma-interferon (IFN-γ)-based blood tests to determine whether or not a higher-than-expected proportion of positive tuberculin skin tests (TSTs) were due to tuberculosis infection. Study design When an unexpectedly high proportion of children in a high school in Leeds were found to have positive TSTs, a cohort study was undertaken based on blood tests and long-term follow-up of the affected children. IFN-γ-based blood tests are reported to be more specific for tuberculosis infection than TSTs. Methods One hundred and ninety children, aged 13–14 years, were screened and 28 (15%) had a positive TST. None had any known risk factor for tuberculosis infection. Parental consent was requested for testing with QuantiFERON-TB Gold (Cellestis, Carnegie, Victoria, Australia). Active cases of tuberculosis with any possible connection to the school or the children were sought through the routine diagnosis and reporting service over the next 36 months. Results Consent was given for 26 children with Heaf Grade 2 results to be tested using QuantiFERON-TB Gold, and blood was obtained from 24 of these children. All tested negative. None of these children developed active tuberculosis, and no cases of active tuberculosis were identified with any connection to the children or the school. Conclusion QuantiFERON-TB Gold testing appeared to identify false-positive TSTs correctly in this group. This supports the recent recommendation to use IFN-γ-based blood tests in people with positive TSTs to confirm or refute the diagnosis of tuberculosis infection.