A clinical research institute follows a dynamic cohort of patients with heart failure with… — USMLE Step 3

A clinical research institute follows a dynamic cohort of patients with heart failure with preserved ejection fraction (HFpEF) to determine the incidence rate of new-onset atrial fibrillation (AF). The total observation period for the study is exactly $4$ years.

Enrollment occurs in three phases:
- $400$ patients are enrolled at the start of the study (Year $0$ ).
- $200$ patients are enrolled exactly $1$ year into the study.
- $150$ patients are enrolled exactly $2$ years into the study.

During the $4$ -year observation period, the following events are recorded:
- $60$ patients from the Year $0$ cohort develop AF exactly $2$ years into the study.
- $30$ patients from the Year $1$ cohort die from non-cardiac causes exactly $2$ years into the study.
- $40$ patients from the Year $2$ cohort are lost to follow-up exactly $2.5$ years into the study.
- $50$ patients from the Year $0$ cohort develop AF exactly $3$ years into the study.

Assume no other patients develop AF, die, or are lost to follow-up, and all remaining patients are followed continuously until the end of the $4$ -year study period.

Calculate the incidence rate of atrial fibrillation for this dynamic cohort in cases per $1,000$ person-years. Round your final answer to the nearest whole integer.

Answer: 50 cases/1,000 PY

Answer

The incidence rate (incidence density) is calculated by dividing the total number of incident (new) cases by the total person-time at risk. In this dynamic cohort, patients enter at different times and exit due to disease onset, death, loss to follow-up, or study completion. Accurately tracking each subgroup yields 110 cases over 2,210 person-years, equating to approximately 50 cases per 1,000 person-years.

Step-by-Step Solution

Determine the numerator (total incident cases of AF).

110 cases

Only new diagnoses of the disease of interest (AF) count as incident cases. The 60 and 50 patients from the Year 0 cohort developed AF. The deaths and lost to follow-up patients do not count towards the numerator.

Calculate person-years (PY) for the Year 0 cohort (400 patients).

1,430 PY

Patients contribute time until they develop the disease or the study ends. 60 patients developed AF at 2 years (120 PY). 50 patients developed AF at 3 years (150 PY). The remaining 290 patients were followed for the full 4 years (1,160 PY). 120 + 150 + 1160 = 1,430.

Calculate person-years for the Year 1 cohort (200 patients).

540 PY

These patients started at Year 1, so their maximum possible follow-up is 3 years. 30 died at Year 2 (1 year of follow-up = 30 PY). The remaining 170 were followed to the end of the study at Year 4 (3 years of follow-up = 510 PY). 30 + 510 = 540.

Calculate person-years for the Year 2 cohort (150 patients).

240 PY

These patients started at Year 2, so their maximum possible follow-up is 2 years. 40 were lost at Year 2.5 (0.5 years of follow-up = 20 PY). The remaining 110 were followed to the end of the study at Year 4 (2 years of follow-up = 220 PY). 20 + 220 = 240.

Calculate the total person-years for the entire dynamic cohort.

2,210 PY

Summing the person-years from all three entry groups: 1,430 + 540 + 240 = 2,210 PY.

Calculate the incidence rate per 1,000 person-years.

Rate = (Cases / Total PY) * 1,000. (110 / 2,210) * 1,000 = 49.7737. Rounded to the nearest whole integer, the answer is 50.

Key Concept

Incidence Density Rate (Person-Time Calculation in Dynamic Cohorts)

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