Incidence and Prevalence
Incidence and prevalence are terms commonly used in describing disease epidemiology.
Incidence is the rate of new (or newly diagnosed) cases of the disease. It is generally reported as the number of new cases occurring within a period of time (e.g., per month, per year). It is more meaningful when the incidence rate is reported as a fraction of the population at risk of developing the disease (e.g., per 100,000 or per million population). Obviously, the accuracy of incidence data depends upon the accuracy of diagnosis and reporting of the disease. In some cases (including ESRD) it may be more appropriate to report the rate of treatment of new cases since these are known, whereas the actual incidence of untreated cases is not.
Incidence rates can be further categorized according to different subsets of the population – e.g., by gender, by racial origin, by age group or by diagnostic category.
Prevalence is the actual number of cases alive, with the disease either during a period of time (period prevalence) or at a particular date in time (point prevalence). Period prevalence provides the better measure of the disease load since it includes all new cases and all deaths between two dates, whereas point prevalence only counts those alive on a particular date.
Prevalence is also most meaningfully reported as the number of cases as a fraction of the total population at risk and can be further categorized according to different subsets of the population.
Incidence to Prevalence
The relationship between incidence and prevalence depends greatly on the natural history of the disease state being reported. In the case of an influenza epidemic, the incidence may be high but not contribute to much growth of prevalence because of the high, spontaneous rate of disease resolution. In the case of a disease that has a low (or zero) cure rate, but where maintenance treatment permits sustained survival, then incidence contributes to continuous growth of prevalence. In such cases, the limitation on prevalence growth is the mortality which occurs in the population. Obviously, prevalence will continue to grow until mortality equals or exceeds the incidence rate.
An example of this relationship is shown below. The disease incidence is 100 per year. Mortality rate is 20% per year. As seen, prevalence grows until the death rate equals the incidence.
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