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Difficulty: MediumMeasures of Disease Frequency

A preventive medicine physician is analyzing data from a 1010-year prospective cohort study evaluating the development of chronic silicosis among miners. At the beginning of the study, 200200 miners were enrolled, of whom 4040 already had a confirmed diagnosis of chronic silicosis at baseline. The remaining disease-free miners were followed for up to 1010 years. During the study period, the following events occurred among the at-risk baseline group:

- 4040 miners developed chronic silicosis exactly 2.52.5 years into the study.
- 6060 miners left the mining industry and were lost to follow-up exactly 55 years into the study without developing silicosis.
- 6060 miners completed the entire 1010-year follow-up period without developing silicosis.

Based on these data, which of the following is the best estimate of the incidence rate of chronic silicosis in this cohort?

  1. A
    2525 per 1,0001,000 person-years
  2. B
    28.628.6 per 1,0001,000 person-years
  3. 4040 per 1,0001,000 person-yearsAnswer
  4. D
    200200 per 1,0001,000 person-years
  5. E
    250250 per 1,0001,000 person-years

Answer

The correct estimate is 4040 per 1,0001,000 person-years.
Incidence rate, also known as incidence density, measures the speed at which new cases of a disease occur within a population. The denominator for this calculation is person-time. First, the 40 miners with baseline silicosis are excluded, leaving 160 at-risk individuals. The person-time is the sum of the observation time for these 160 individuals: 40 people contributed 2.5 years each (100 person-years), 60 people contributed 5 years each (300 person-years), and 60 people contributed 10 years each (600 person-years). This results in a total of 1,000 person-years. Dividing the 40 new cases by the 1,000 person-years yields an incidence rate of 40 per 1,000 person-years.

Step-by-Step Solution

1
Determine the population at risk at the start of the study.
Total enrolled (200200) minus those with the disease at baseline (4040) equals 160160 miners at risk.
Prevalent cases cannot develop the disease again and must be excluded from incidence calculations.
2
Calculate the total person-time contributed by the at-risk cohort.
(4040 cases × 2.52.5 years) + (6060 lost × 55 years) + (6060 completed × 1010 years) = 100100 + 300300 + 600600 = 1,0001,000 person-years.
Person-time requires summing the exact duration each at-risk individual remained in the study before developing the disease or being censored.
3
Calculate the incidence rate.
4040 new cases / 1,0001,000 person-years = 0.040.04 per person-year, which is 4040 per 1,0001,000 person-years.
Incidence rate (incidence density) is the number of new cases divided by the total person-time at risk.

Key Concept

Incidence rate (incidence density) is calculated by dividing the number of new cases by the total person-time accumulated by the at-risk population.
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Topics

Measures of Disease Frequency

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