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Difficulty: HardMeasures of Disease Frequency

A health system is evaluating the incidence rate of first hospital readmission among patients enrolled in a novel outpatient heart failure management program over a 1212-month study period (January 1 to December 31). Patients are followed from their exact date of enrollment until their first hospital readmission or until the end of the study period, whichever comes first.

A total of 600600 patients were enrolled in the program in two distinct cohorts:
- Cohort A (300300 patients) was enrolled exactly on January 1. Of these, 240240 patients completed the full 1212 months without a readmission. The remaining 6060 patients had their first readmission exactly at the 66-month mark of their enrollment.
- Cohort B (300300 patients) was enrolled exactly on July 1. Of these, 270270 patients completed their full available 66-month follow-up period (until December 31) without a readmission. The remaining 3030 patients had their first readmission exactly at the 33-month mark of their enrollment.

Based on these data, what is the incidence density (incidence rate) of first hospital readmission per 1,0001,000 person-months?

  1. A
    12.512.5
  2. B
    13.713.7
  3. C
    16.716.7
  4. 18.218.2Answer
  5. E
    150.0150.0

Answer

The correct incidence rate is 18.218.2 per 1,0001,000 person-months.
The correct calculation for incidence density (incidence rate) requires dividing the total number of new cases by the exact total person-time at risk. Cohort A contributes 3,2403,240 person-months (240×12240 \times 12 plus 60×660 \times 6). Cohort B, starting midway through the year, can only contribute a maximum of 66 months per person; they contribute 1,7101,710 person-months (270×6270 \times 6 plus 30×330 \times 3). The total person-time is 4,9504,950 person-months. Dividing the 9090 total cases by 4,9504,950 person-months yields 0.018180.01818 cases per person-month, or 18.218.2 cases per 1,0001,000 person-months.

Step-by-Step Solution

1
Calculate the total person-months contributed by Cohort A.
Cohort A = 3,2403,240 person-months.
240240 patients were followed for 1212 months (240×12=2,880240 \times 12 = 2,880). 6060 patients were followed for 66 months before readmission (60×6=36060 \times 6 = 360). 2,880+360=3,2402,880 + 360 = 3,240.
2
Calculate the total person-months contributed by Cohort B.
Cohort B = 1,7101,710 person-months.
270270 patients were followed for 66 months (270×6=1,620270 \times 6 = 1,620). 3030 patients were followed for 33 months before readmission (30×3=9030 \times 3 = 90). 1,620+90=1,7101,620 + 90 = 1,710.
3
Calculate the total person-time at risk for the entire study population.
4,9504,950 total person-months.
Sum the person-time from Cohort A and Cohort B (3,240+1,710=4,9503,240 + 1,710 = 4,950).
4
Calculate the incidence rate (incidence density) per 1,000 person-months.
18.218.2 per 1,0001,000 person-months.
Total incident cases = 60+30=9060 + 30 = 90. Incidence Rate = (90/4,950)×1,000=18.18(90 / 4,950) \times 1,000 = 18.18, which rounds to 18.218.2.

Key Concept

Incidence density (incidence rate) in a dynamic cohort must account for both delayed entry times and early exits (censoring) when calculating the total person-time denominator.
Estimated Time:2m 30s
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Topics

Measures of Disease Frequency

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