A primary care network utilizes its electronic health record registry to track the develop… — USMLE Step 3

A primary care network utilizes its electronic health record registry to track the development of chronic kidney disease (CKD) among adult patients with essential hypertension. At the beginning of a $5$ -year observation period, $1,200$ patients with hypertension are identified. A baseline laboratory screening reveals that $100$ of these patients already have CKD; they are excluded from the at-risk cohort.

The remaining patients are followed prospectively for exactly $5$ years. The registry data reveal the following:
- $100$ patients develop CKD exactly $2$ years into the observation period.
- $100$ patients transfer their care to an out-of-network provider and are lost to follow-up exactly $3$ years into the study (without a CKD diagnosis prior to transfer).
- The remaining $900$ patients complete the full $5$ -year follow-up without developing CKD.

Based on this registry data, what is the incidence rate of CKD in this cohort?

A
$18.2$ per $1,000$ person-years
B
$19.2$ per $1,000$ person-years
$20.0$ per $1,000$ person-yearsAnswer
D
$33.3$ per $1,000$ person-years
E
$90.9$ per $1,000$ person-years

Answer

The correct answer is

20.0

per

1,000

person-years.

The incidence rate (incidence density) measures the number of new cases of a disease divided by the total person-time at risk. First, we determine the at-risk population by subtracting the

100

prevalent cases from the

1,200

initial patients, leaving

1,100

at-risk patients. Next, we calculate the total person-years (PY):

100

patients develop CKD at

2

years (

200

PY);

100

patients are lost to follow-up at

3

years (

300

PY); and

900

patients complete the

5

years disease-free (

4,500

PY). The total PY is

200 + 300 + 4,500 = 5,000

PY. Finally, the incidence rate is

100

new cases divided by

5,000

PY, which equals

0.02

cases per PY, or

20.0

per

1,000

person-years.

Step-by-Step Solution

Identify the true at-risk population at the start of the observation period.

1,200

total patients -

100

prevalent cases =

1,100

at-risk patients.

Patients who already have the disease at baseline cannot become incident cases and must be excluded from the denominator.

Calculate the total person-years (PY) contributed by each subgroup during the study.

Incident cases:

100 \times 2 = 200

PY. Lost to follow-up:

100 \times 3 = 300

PY. Completers:

900 \times 5 = 4,500

PY. Total =

5,000

PY.

Incidence rate requires knowing the exact time each individual was at risk of developing the disease before either getting it, dropping out, or completing the study.

Calculate the incidence rate by dividing the number of new cases by the total person-years.

100

new cases /

5,000

PY =

0.02

cases per PY. Multiplying by

1,000

gives

20.0

per

1,000

person-years.

This formula provides the incidence density, reflecting the true rate at which new cases occur over time.

Key Concept

Calculating incidence rate (density) requires dividing the number of new disease cases by the exact person-time contributed by the at-risk population, excluding prevalent cases.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic