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Difficulty: Very hardMeasures of Disease Frequency

An institutional review board is auditing the results of a prospective cohort study investigating a novel biomarker for systemic lupus erythematosus (SLE) progression. The study initially enrolled 1,0001,000 asymptomatic patients with positive antinuclear antibodies. The protocol followed the cohort over a strict 55-year period to observe the development of overt clinical SLE.

The longitudinal data logs reveal the following timeline:
- At the end of Year 1, 100100 patients developed overt clinical SLE.
- At the end of Year 2, 100100 patients relocated and were lost to follow-up (confirmed disease-free at the time of relocation).
- At the end of Year 3, another 100100 patients developed overt clinical SLE.
- At the end of Year 4, 100100 patients withdrew from the study (confirmed disease-free at the time of withdrawal).
- The remaining patients completed the full 55-year follow-up without developing the disease.

Based on this data, what is the incidence rate of overt clinical SLE in this cohort, expressed per 1,0001,000 person-years?

  1. A
    4040 per 1,0001,000 person-years
  2. 5050 per 1,0001,000 person-yearsAnswer
  3. C
    5959 per 1,0001,000 person-years
  4. D
    200200 per 1,0001,000 person-years
  5. E
    250250 per 1,0001,000 person-years

Answer

The incidence rate is 5050 per 1,0001,000 person-years.
The correct answer accurately calculates the incidence density by dividing the total number of new cases (200200) by the exact total person-years (PY) contributed by the cohort. By mapping the cohort's timeline: 100100 patients contributed 11 year (100100 PY), 100100 contributed 22 years (200200 PY), 100100 contributed 33 years (300300 PY), 100100 contributed 44 years (400400 PY), and the remaining 600600 contributed the full 55 years (3,0003,000 PY). The sum is 4,0004,000 PY. Dividing 200200 cases by 4,0004,000 PY yields 0.050.05 per person-year, which converts to 5050 per 1,0001,000 person-years.

Step-by-Step Solution

1
Determine the total number of incident cases.
100100 cases (Year 1) + 100100 cases (Year 3) = 200200 total incident cases.
The numerator for incidence rate requires all new cases developed during the study period.
2
Calculate the person-years (PY) contributed by individuals who developed the disease.
(100100 patients × 11 year) + (100100 patients × 33 years) = 100+300=400100 + 300 = 400 PY.
Patients contribute risk time only up until the moment they develop the disease.
3
Calculate the person-years contributed by individuals lost to follow-up or withdrawn.
(100100 patients × 22 years) + (100100 patients × 44 years) = 200+400=600200 + 400 = 600 PY.
Censored individuals contribute risk time up to their exit from the study.
4
Calculate the person-years contributed by patients who completed the study disease-free.
1,0001,000 starting - 200200 diseased - 200200 withdrawn/lost = 600600 completing patients. (600600 patients × 55 years) = 3,0003,000 PY.
These individuals remained at risk for the full 5-year duration of the study.
5
Sum all person-years to find the total denominator and calculate the rate.
400+600+3,000=4,000400 + 600 + 3,000 = 4,000 PY. Rate = 200200 cases / 4,0004,000 PY = 0.050.05 cases per PY, or 5050 per 1,0001,000 PY.
Incidence rate (incidence density) divides the total incident cases by the exact accumulated person-time of observation.

Key Concept

Incidence Rate (Incidence Density) calculation in a dynamic cohort
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Topics

Measures of Disease Frequency

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