Interferon-gamma release assays and TB diagnosis

Worldwide in 2007, there were an estimated 9.27 million new cases of active tuberculosis (TB) and 1.8 million deaths related to the disease. However, in most people with TB, the infection is 'latent'; the underlying cause, Mycobacterium tuberculosis, remains alive but patients are asymptomatic and non-infectious. Latent infection progresses to active disease in around 10% of individuals at some point. It therefore represents a large reservoir of potential future active cases who, if correctly identified, can be treated with prophylactic anti-tuberculosis chemotherapy before active TB develops. Until recently, the only test to indicate the relative risk of an individual having latent TB infection was the tuberculin skin test (TST), but this lacks specificity.3 Here we look at the place of interferon-gamma release assays (IGRAs), a new approach in the diagnosis of TB.