An epidemiological study is initiated to assess the impact of type II diabetes mellitus in a large population with little migration. Disease incidence and prevalence are monitored and recorded as the number of cases per 1,000 individuals over a 30-year period. Which of the following is the most likely explanation for the change in disease prevalence seen in the graph?

Question

An epidemiological study is initiated to assess the impact of type II diabetes mellitus in a large population with little migration.  Disease incidence and prevalence are monitored and recorded as the number of cases per 1,000 individuals over a 30-year period.  Which of the following is the most likely explanation for the change in disease prevalence seen in the graph?

Quizplus · Accepted Answer

Incidence and prevalence are important concepts for evaluating the impact of disease on a population. Incidence corresponds to the number of new cases of a disease diagnosed in a population at risk over a given period. Prevalence refers to the total number of diseased individuals in the population at a particular point; it is dependent on incidence and the average duration of the disease (ie, time from diagnosis to cure or death): Prevalence = (Incidence) x (Duration of disease) A simplified diagram compares incidence and prevalence to adding new drops (incident cases) to a sink collecting water (prevalent cases). The above graph shows that the incidence of type II diabetes mellitus has been constant for the last 30 years (Choice F), but that prevalence is rising. Because this is a steady-state population with little migration, a constant incidence rate should result in a nearly stable disease prevalence, unless there is some additional factor affecting the duration of the condition. Factors that prolong disease duration can increase disease prevalence even when the incidence is unchanged because diseased individuals live longer on average. In this case, improved quality of care likely led to a lower mortality rate in diabetics over time, resulting in a higher prevalence of the disease in the population (Choice B). (Choice A) A decreased hospitalization rate would indicate reduced morbidity (ie, burden of disease) among diabetic individuals. However, a decrease in hospitalization rate would not necessarily be associated with reduced mortality and increasing disease prevalence. (Choice D) Increased accuracy of diagnostic testing would affect both the prevalence and incidence of a disease as it changes the rate at which new cases are detected. (Choice E) Increased exposure to diabetes risk factors would likely increase both disease incidence and prevalence. (Choice G) Selective survival bias occurs in case-control studies when cases are selected from the entire disease population instead of from just those that are newly diagnosed. For instance, a study on cancer survival that is not limited to newly diagnosed patients could contain a higher proportion of relatively benign malignancies, as these patients generally live longer. Educational objective: Prevalence equals the incidence rate multiplied by the average disease duration. Changing disease prevalence in a steady-state population with a constant incidence rate means that there is an additional factor affecting the duration of the condition. A factor that prolongs disease duration (eg, improved quality of care) will increase disease prevalence, as affected patients survive longer.

An Epidemiological Study Is Initiated to Assess the Impact of Type