Before implementation of a programme, its cost–effectiveness, or cost-benefit – in which costs and benefits are compared in monetary terms – can be modelled from estimates of the predicted number of additional true-positive TB patients detected, the reduction in morbidity, the reduction in the time that a person remains infectious and reductions in transmission, incidence and mortality. Cost will depend on the NNS, the algorithm used for screening and diagnosis, the method used to reach people for screening and the direct and indirect costs incurred by the screened individuals.
Models can be used to estimate how costs are related to the potential impact on TB transmission and epidemiology. Some empirical evidence on the impact of active community-wide screening on the prevalence of tuberculosis is beginning to emerge (26). The ScreenTB tool can be used as a basic calculator of cost per case detected through screening (see 3.3).