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Difficulty: MediumMeasures of Disease Frequency and Association

A team of clinical epidemiologists is conducting a study on the development of chronic kidney disease (CKD) among patients with type 2 diabetes mellitus at a large regional health center. On January 1, the health center's registry includes 5,0005,000 patients with type 2 diabetes. Cross-sectional baseline screening reveals that 1,0001,000 of these patients already meet the diagnostic criteria for CKD.

Over the next 55 years, the researchers prospectively follow the remaining patients. By the end of the 5-year period, they identify 200200 new diagnoses of CKD in this group. During the study, 150150 patients die from severe cardiovascular complications.

Assuming no patients are lost to follow-up for other reasons, which of the following best represents the cumulative incidence of CKD in this cohort over the 5-year period?

  1. A
    4%4\%
  2. B
    24%24\%
  3. 5%5\%Answer
  4. D
    25%25\%
  5. E
    20%20\%

Answer

The cumulative incidence of CKD in this cohort over the 5-year period is 5%5\%.
The correct answer is 5%5\%. Cumulative incidence measures the proportion of people who develop a new disease during a specified time period among those who are at risk at the beginning of the period. The population at risk excludes those who already have the disease. In this scenario, out of 5,0005,000 total patients, 1,0001,000 already have CKD. Therefore, the population at risk is 4,0004,000. Over the 5-year period, 200200 new cases occur. The cumulative incidence is calculated as 2004,000=0.05\frac{200}{4,000} = 0.05, which is 5%5\%. Deaths that occur during the follow-up period do not retroactively change the initial population at risk for a cumulative incidence calculation.

Step-by-Step Solution

1
Determine the population at risk at the start of the study.
5,000−1,000=4,0005,000 - 1,000 = 4,000 patients at risk.
Incidence only measures new cases among those who do not already have the disease. Therefore, prevalent cases must be subtracted from the total population.
2
Identify the number of new (incident) cases that developed during the time period.
200200 new cases of CKD.
The numerator for incidence is strictly the number of newly diagnosed cases.
3
Calculate the cumulative incidence.
2004,000=0.05=5%\frac{200}{4,000} = 0.05 = 5\%.
Cumulative incidence is the number of new cases divided by the population at risk.

Key Concept

Incidence vs. Prevalence calculations
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Topics

Measures of Disease Frequency and Association

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