An incidence estimation model for multi-stage diseases with differential mortality.

Prevalence and incidence are two important measures of the impact of a disease. For many diseases, incidence is the most useful measure for response planning. However, the longitudinal studies needed to calculate incidence are resource-intensive, so prevalence estimates are often more readily available. In 1986, Podgor and Leske (Statistics in Medicine, 5:573-578, 1986) developed a model to estimate incidence of a single disease from one survey of age-specific prevalence, even where the presence of the disease increases the mortality rate of patients. Here, we extend their model to the case of progressive diseases, where the incidence of all disease stages is desired. As an example, we consider the case of cataract disease in Africa, where ophthalmologists wish to distinguish between unilateral and bilateral cataract incidence in order to plan the number of cataract surgeries needed to prevent the occurrence of blindness as a result of the disease. Our method has successfully provided cataract incidence estimates on the basis of prevalence data from new Rapid Assessment of Avoidable Blindness surveys in Africa (Lewallen et al., Archives of Ophthalmology, 128(12):1584-1589, 2010). In this paper, we provide a more general form of the model in order to promote its applicability to other diseases.