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Difficulty: EasyMeasures of Disease Frequency

A local health department is investigating a new respiratory infection in a retirement community. On January 1, the community has a total population of 5,000 residents. Initial screening on that day reveals that 200 residents already have the infection. The remaining residents are followed for the next 12 months. During this 1-year period, 480 new cases of the infection are diagnosed. No residents die or leave the community during the year. What is the cumulative incidence of this infection in the community over the 1-year period?

  1. A
    4.0%
  2. B
    9.6%
  3. 10.0%Answer
  4. D
    13.6%
  5. E
    14.2%

Answer

10.0%
The correct answer accurately calculates the cumulative incidence. Cumulative incidence is defined as the number of new cases of a disease during a specific time period divided by the population initially at risk. In this scenario, the initial population is 5,000. However, 200 residents already have the disease, so they are not at risk of acquiring it. The population at risk is therefore 5,000 - 200 = 4,800. There were 480 new cases. The calculation is 480 / 4,800 = 0.10, which equals 10.0%.

Step-by-Step Solution

1
Determine the population at risk at the beginning of the study.
5,000 total residents - 200 residents already infected = 4,800 residents at risk.
Individuals who already have the disease cannot become 'new' cases and must be excluded from the denominator.
2
Identify the number of new cases during the time period.
480 new cases.
The numerator for cumulative incidence only includes newly diagnosed cases during the specified time period.
3
Calculate cumulative incidence.
480 / 4,800 = 0.10, or 10.0%.
Cumulative incidence is the ratio of new cases to the population at risk over a specified time period.

Key Concept

Measures of Disease Frequency: Cumulative Incidence
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Topics

Measures of Disease Frequency

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