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Difficulty: EasyMeasures of Disease Frequency

A clinical researcher is evaluating the development of chronic kidney disease (CKD) in a cohort of 1,2001,200 factory workers. At the initial baseline screening, 200200 workers are found to already have CKD and are referred to a nephrologist. The remaining workers are enrolled in the study and followed over a 55-year period. During this follow-up period, 5050 of the enrolled workers are newly diagnosed with CKD. Based on these data, what is the cumulative incidence of CKD in this cohort over the 55-year study period?

  1. A
    4.2%4.2\%
  2. 5.0%5.0\%Answer
  3. C
    16.7%16.7\%
  4. D
    20.8%20.8\%
  5. E
    25.0%25.0\%

Answer

The correct cumulative incidence is 5.0%5.0\%.
The correct answer accurately calculates the cumulative incidence. To find the cumulative incidence, one must divide the number of new cases (5050) by the population strictly at risk for developing the disease. The population at risk is the total initial population minus those who already had the disease at baseline (1,200−200=1,0001,200 - 200 = 1,000). Therefore, the cumulative incidence is 50/1,00050 / 1,000, which equals 0.050.05 or 5.0%5.0\%.

Step-by-Step Solution

1
Determine the population at risk at the start of the study.
Population at risk = Total population - Individuals who already have the disease = 1,200−200=1,0001,200 - 200 = 1,000.
Cumulative incidence only measures new cases among those who are capable of developing the disease. Those who already have the disease must be excluded from the denominator.
2
Identify the number of new (incident) cases during the study period.
New cases = 5050.
The problem states that 5050 enrolled workers were newly diagnosed during the 55-year follow-up.
3
Calculate the cumulative incidence.
Cumulative incidence = (New cases / Population at risk) = 50/1,000=0.0550 / 1,000 = 0.05, which is 5.0%5.0\%.
This formula provides the proportion of a population at risk that develops the disease over a specified time period.

Key Concept

Cumulative incidence is calculated by dividing the number of new cases of a disease during a specific time period by the total population at risk at the beginning of that period.
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Topics

Measures of Disease Frequency

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