A prospective cohort study is conducted to evaluate the incidence of major adverse cardiov… — USMLE Step 3

A prospective cohort study is conducted to evaluate the incidence of major adverse cardiovascular events (MACE) in patients receiving a novel lipid-lowering therapy. The investigators enroll $1,000$ patients who are free of cardiovascular disease at baseline. The study follows these patients for exactly $4$ years. During the study period, the following events are recorded:

- $200$ patients experience a MACE at exactly the end of Year $1$ .
- $100$ patients are lost to follow-up (without experiencing a MACE) at exactly the end of Year $2$ .
- The remaining $700$ patients complete the full $4$ -year follow-up without experiencing a MACE.

Based on these data, what is the incidence rate of MACE per $1,000$ person-years in this cohort?

Answer: 62.5 cases per 1,000 person-years

Answer

62.5

To calculate the incidence rate per

1,000

person-years, the number of incident cases (

200

) must be divided by the exact person-time at risk for the entire cohort. The

200

patients who had an event at Year

1

contribute

200

person-years. The

100

patients lost at Year

2

contribute

200

person-years. The

700

patients completing the

4

-year study contribute

2,800

person-years. The total person-time is

3,200

person-years. Dividing the

200

cases by

3,200

person-years yields an incidence rate of

0.0625

cases per person-year. Multiplying this by

1,000

gives exactly

62.5

cases per

1,000

person-years.

Step-by-Step Solution

Determine the number of incident cases.

200

cases.

Only the

200

patients who experienced a MACE are counted in the numerator for incidence.

Calculate the person-time for patients who developed a MACE.

200 \times 1 = 200

person-years.

These patients stopped contributing time at risk exactly at the end of Year

1

when the event occurred.

Calculate the person-time for patients lost to follow-up.

100 \times 2 = 200

person-years.

These patients were followed for

2

years before dropping out, remaining event-free during that time.

Calculate the person-time for patients who completed the study.

700 \times 4 = 2,800

person-years.

The remaining

700

patients were followed for the entire

4

-year duration without an event.

Calculate total person-years and the raw incidence rate.

200 + 200 + 2,800 = 3,200

person-years. Rate =

\frac{200}{3200} = 0.0625

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

Convert to the requested unit (per

1,000

person-years).

0.0625 \times 1,000 = 62.5

The question specifically asks for the rate per

1,000

person-years.

Key Concept

Incidence rate (person-time incidence) accurately accounts for differing lengths of follow-up by dividing the total number of new disease cases by the sum of the time periods that all individuals were observed and at risk.

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