New tests will improve detection of latent TB

In the UK cases of active TB have risen substantially over the past 20 years. This increase has occurred almost exclusively in individuals born outside the UK, who now constitute more than two-thirds of cases. Only around one in ten people who are infected will develop active disease. The remaining 90% are presumed to have latent TB infection (LTBI) where viable mycobacteria are thought to persist for decades, and may reactivate if the host's immune system is weakened. In a country such as the UK with a low incidence of TB, a high proportion of cases result from reactivation of latent TB, rather than transmission by infectious cases. In the past 10 years a novel type of diagnostic test for LTBI has been developed: the interferon-gamma release assays (IGRA). Their major advantage over the tuberculin skin test is that they are not affected by prior BCG vaccination and they have a specificity of well over 90%. These tests are unable to distinguish between active and latent TB infection: this distinction must be performed purely on clinical grounds. Patients with a positive test should be assessed by a clinician with expertise in TB to ensure an appropriate management plan for each patient. The role of IGRAs in diagnosis of active TB is limited since in a patient with suspected active TB a positive result may indicate LTBI in combination with an alternative diagnosis. At a population level screening and chemoprophylaxis contributes usefully to TB control. However, only those under 35 with LTBI should receive prophylaxis. After this age the increasing risks of hepatotoxicity begin to outweigh the diminishing benefits of prophylaxis. The exceptions are healthcare workers, where the benefits are not just to the individual but also extend to their patients, and immunocompromised patients. The IGRAs represent a major development in the diagnosis of LTBI. While currently most of their use is through established TB screening services, it is likely in future that they will also be used routinely in general practice to screen individuals at high risk of LTBI.