Neither the tuberculin skin test nor interferon-gamma release assays (IGRAs) are, however, curently capable of predicting reliably the risk of future tuberculosis as this study from the Gambia among contacts of sputum smear-positive tuberculosis cases demonstrates: the relative superiority in predicting the development of tuberculosis of IGRAs over the tuberculin skin test might be considerable, but the absolute difference is minuscule: both the tuberculin skin test and IGRAs measure an immunological response resulting from prior infection with M tuberculosis but they cannot currently distinguish between viable tubercle bacilli residing in the host versus an immunological memory nor is this a measure of the competence of the host's immune system to prevent progression from latent infection with M tuberculosis to tuberculosis.
