A state's population for 2016 was estimated at 4,000,000. At the beginning of the same year, the state reported having 20,000 cases of a rare and fatal neurodegenerative condition. At the end of 2016, the state reported 7,000 new cases, as well as 1,000 deaths attributable to this condition. Another 40,000 deaths from all other causes were also reported at the end of the year. What was the cumulative incidence of the disease in 2016?

Question

A state's population for 2016 was estimated at 4,000,000.  At the beginning of the same year, the state reported having 20,000 cases of a rare and fatal neurodegenerative condition.  At the end of 2016, the state reported 7,000 new cases, as well as 1,000 deaths attributable to this condition.  Another 40,000 deaths from all other causes were also reported at the end of the year.  What was the cumulative incidence of the disease in 2016?

Quizplus · Accepted Answer

Cumulative incidence (CI) is calculated as the total number of new cases of a disease (eg, neurodegenerative condition) over a specific period (eg, during 2016) divided by the number of people at risk at the beginning of the period. It does not account for deaths in the period under study because the patients who died were, at some point in the intervening period, still considered part of the baseline population at risk for the disease. In this case, there were 3,980,000 people at risk (ie, 4,000,000 − 20,000 already with the disease) at the beginning of 2016, and 7,000 new cases of the disease occurred over the 1-year period being analyzed. Therefore, the CI is: CI = 7,000 / (4,000,000 − 20,000) = 7,000 / 3,980,000 Unlike incidence rate (which is typically reported as the number of cases per person-year), CI does not incorporate time into the denominator; it is simply the proportion of at-risk individuals who contract the disease over the specified period. (Choice A) The disease-specific mortality (in this example: 1,000 / 4,000,000) is the number of deaths attributable to a disease per year divided by the total population. (Choice B) The CI does not account for deaths in the period under study. In this case, the numerator is 7,000, not 6,000. (Choice C) The rate of increase of a disease (ie, the increase in disease prevalence) is the number of new cases per year minus the number of deaths (or cures) attributable to the disease per year, divided by the total population. This yields (7,000 − 1,000) / 4,000,000. (Choice E) The CI is calculated by taking into consideration the population at risk at the beginning of the period under study. In this case, the denominator must be reduced by the number of cases reported at the beginning of the year 2016 (ie, 4,000,000 − 20,000 = 3,980,000). (Choice F) The prevalence of disease at the end of 2016 takes into consideration the prevalent cases at the beginning of the year, the incident cases throughout the year, and all the deaths reported by the end of the year. In this case, the prevalence at the closing of 2016 is: ([20,000 + 7,000] - 1,000) / (4,000,000 - [40,000 + 1,000]). (Choice G) The mortality rate is the number of deaths per year divided by the total population. This yields 41,000 / 4,000,000. Educational objective: The cumulative incidence of a disease is the number of new cases of a disease over a specific period divided by the total population at risk at the beginning of the study (ie, the proportion of at-risk individuals who contract the disease over the specified period).

A State's Population for 2016 Was Estimated at 4,000,000