A prospective cohort study is conducted to evaluate the risk of developing chronic obstruc… — USMLE Step 3

A prospective cohort study is conducted to evaluate the risk of developing chronic obstructive pulmonary disease (COPD) among $500$ healthy adult smokers. The study follows the participants for exactly $4$ years. During the follow-up period, the following events are recorded:

- $300$ participants remain free of COPD and complete the entire $4$ -year follow-up.
- $40$ participants develop COPD exactly $1$ year after study enrollment.
- $40$ participants develop COPD exactly $2$ years after study enrollment.
- $80$ participants move away and are lost to follow-up exactly $2$ years after study enrollment (none had COPD at their last visit).
- $40$ participants die of unrelated cardiovascular events exactly $3$ years after study enrollment (none had COPD prior to death).

What is the incidence rate of COPD in this cohort, expressed as cases per $100$ person-years?

Answer: 5 cases per 100 person-years

Answer

The incidence rate represents the speed at which new cases of a disease occur in a population at risk. It is calculated as the total number of new cases divided by the sum of the person-time contributed by all individuals during the observation period. In this cohort, 80 incident cases occurred over 1600 person-years of follow-up. Dividing 80 by 1600 yields 0.05 cases per person-year, which converts to 5 cases per 100 person-years.

Step-by-Step Solution

Determine the total number of incident cases.

80 cases.

40 patients developed COPD at 1 year and 40 developed it at 2 years.

Calculate the total person-years contributed by each group of participants.

1200 + 40 + 80 + 160 + 120 = 1600 total person-years.

Participants contribute time only until they develop the disease, drop out, die, or complete the study.

Calculate the incidence rate per person-year.

80 cases / 1600 person-years = 0.05 cases per person-year.

Incidence rate is the number of new cases divided by total person-time at risk.

Convert the incidence rate to cases per 100 person-years.

0.05 * 100 = 5 cases per 100 person-years.

The question specifically requests the rate expressed per 100 person-years.

Key Concept

Incidence Rate (Person-Time Rate)

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