The validity of estimating the incidence of diabetic retinopathy from prevalence

A method for estimating incidence rates of a disease from prevalence proportions with differential mortality was validated for diabetic retinopathy in a large population of diabetic persons followed for 4 years, 1980-1982 to 1984-1986, in southern Wisconsin. Four-year age- or duration-specific incidence rates of retinopathy were calculated from prevalence and mortality in persons with and those without retinopathy. These rates were lower than the actual rates obtained at the 4-year follow-up examination by 50% or more in most age intervals in both younger and older onset persons. Estimated duration-specific rates were up to 29 and 57% lower than the actual rates in younger and older onset persons, respectively. These discrepancies could not be accounted for by violations of the assumptions underlying the method. In applying the method to diabetic retinopathy, age- and duration-specific incidence may be more appropriate. Such estimated rates agree well with actual rates except in older onset persons with diabetes of short duration.