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Difficulty: HardMeasures of Disease Frequency

A team of clinical epidemiologists is investigating the incidence of a newly identified neurodegenerative syndrome in a cohort of retired industrial workers exposed to a specific volatile organic compound.

The researchers initially recruit 1,2001,200 retired workers for a 1010-year prospective study. During the baseline clinical evaluation, 200200 of these individuals are diagnosed with the neurodegenerative syndrome and are excluded from the follow-up. The remaining cohort is monitored closely over the 1010-year period. The surveillance data reveals the following:

- 100100 individuals are newly diagnosed with the neurodegenerative syndrome during the study period.
- 100100 individuals die from unrelated cardiovascular events or are lost to follow-up.
- The remaining 800800 individuals complete the 1010-year observation period without developing the syndrome.

Assuming that all new diagnoses and losses to follow-up occur uniformly throughout the 1010-year study, which of the following best represents the incidence rate of the neurodegenerative syndrome in this cohort?

  1. A
    8.38.3 cases per 1,0001,000 person-years
  2. B
    10.010.0 cases per 1,0001,000 person-years
  3. 11.111.1 cases per 1,0001,000 person-yearsAnswer
  4. D
    25.025.0 cases per 1,0001,000 person-years
  5. E
    33.333.3 cases per 1,0001,000 person-years

Answer

The incidence rate is 11.111.1 cases per 1,0001,000 person-years, calculated by dividing the 100100 new cases by the 9,0009,000 person-years of observation.
The incidence rate (incidence density) measures the number of new cases of a disease per unit of person-time at risk. First, the initial 200200 individuals with the disease are excluded, leaving an at-risk cohort of 1,0001,000. Individuals who complete the study without disease (800800) contribute 1010 years each (8,0008,000 person-years). Those who develop the disease (100100) or are lost to follow-up (100100) uniformly over the 1010 years contribute an average of 55 years each (500+500=1,000500 + 500 = 1,000 person-years). The total person-time is 9,0009,000 person-years. Dividing the 100100 new cases by 9,0009,000 person-years yields approximately 11.111.1 cases per 1,0001,000 person-years.

Step-by-Step Solution

1
Determine the initial at-risk population.
1,2001,200 total recruited - 200200 with baseline disease = 1,0001,000 individuals at risk.
Prevalent cases already have the disease and are not at risk of developing it; they must be excluded from incidence calculations.
2
Calculate person-years (PY) for individuals who completed the study disease-free.
800800 individuals \times 1010 years = 8,0008,000 PY.
These individuals were observed for the entire duration of the study.
3
Calculate person-years for individuals who developed the disease or were lost to follow-up.
(100100 incident cases + 100100 lost to follow-up) \times 55 years = 1,0001,000 PY.
Because these events occurred uniformly over the 1010-year study, they contributed an average of half the study time (55 years each) to the denominator.
4
Calculate the total person-years and the final incidence rate.
Total PY = 8,0008,000 + 1,0001,000 = 9,0009,000 PY. Incidence Rate = 100100 cases / 9,0009,000 PY = 0.01110.0111 cases per PY, or 11.111.1 cases per 1,0001,000 PY.
Incidence rate requires dividing the number of new cases by the total person-time at risk.

Key Concept

Incidence rate (incidence density) requires establishing a disease-free at-risk cohort and accurately summing the specific person-time each individual contributes until disease onset, loss to follow-up, or study completion.
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Topics

Measures of Disease Frequency

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