The deficiencies in the specificity of the tuberculin skin test have been largely overcome by the alternative interferon-gamma release assays (IGRAs). These tests are based on antigens that are specific to M tuberculosis, absent from M bovis BCG and most environmental mycobacteria. These assays provoke the release of interferon-gamma from macrophages in whole blood upon exposure to these antigens if the tested person had had prior infection with M tuberculosis.
This is exemplified here in the difference of the proportion of reactors to tuberculin among BCG-vaccinated and unvaccinated persons given a tuberculin skin test (blue) or one of the two currently commerciallay available IGRAs.
