Annual risk of infection with Mycobacterium tuberculosis

The average annual risk of infection with Mycobacterium tuberculosis is a calculated average from an observed prevalence of infection, approximating the incidence of infection. It has the potential to be informative about the extent of transmission in a community. Where serial surveys are available, secular trends in transmission might be ascertained. The prerequisite for calculating the average annual risk is the successful determination of the prevalence of infection. Difficulties arising with tuberculin skin-test surveys include logistical problems in sampling a representative portion of the population. Therefore, a compromise assesses the prevalence of infection among school children as an indicator of transmission in the community at large. The utilisation of tuberculin surveys is further compounded by the unpredictable specificity of the tuberculin skin test and, thus, the predictive value of a positive test result. Statistical approaches using mixture analysis may overcome this problem to some extent, but the experience is limited. Cytokine-derived assays, such as the interferon-gamma release assays, are promising in providing higher specificity, but they require venepuncture.