A local public health department is investigating the risk of new-onset type 2 diabetes me… — USMLE Step 3

A local public health department is investigating the risk of new-onset type 2 diabetes mellitus in a cohort of night-shift workers at a large manufacturing plant. At the beginning of the prospective study, $315$ workers are enrolled. Baseline screening reveals that $15$ workers already have a diagnosis of type 2 diabetes; they are excluded from the longitudinal follow-up. The remaining workers are followed for $5$ years.

The study yields the following follow-up data:
- $120$ workers complete the entire $5$ -year study without developing diabetes.
- $30$ workers develop diabetes at exactly $24$ months into the study.
- $30$ workers develop diabetes at exactly $48$ months into the study.
- $120$ workers relocate and are lost to follow-up at exactly $42$ months into the study. None of these workers developed diabetes prior to their departure.

Based on the information above, what is the incidence rate of type 2 diabetes in this cohort, expressed per $1,000$ person-years?

A
$38.1$
B
$40.0$
$50.0$ Answer
D
$62.5$
E
$200.0$

Answer

The correct answer is

50.0

per

1,000

person-years.

The correct answer accurately calculates the incidence rate by dividing the number of new cases (

60

) by the precise sum of person-years contributed by the at-risk population (

1,200

). Incidence rate, or incidence density, requires excluding baseline prevalent cases and summing the exact observation time for each individual until disease onset, loss to follow-up, or study end.

Step-by-Step Solution

Determine the at-risk population and the number of incident (new) cases.

At-risk population =

315 - 15 = 300

workers. Total incident cases =

30 + 30 = 60

workers.

Baseline prevalent cases must be excluded from both the numerator and the at-risk denominator when evaluating new disease incidence.

Calculate the total person-years (PY) contributed by the at-risk cohort by summing the observation time for each group.

Group 1 (full follow-up):

120 \text{ workers} \times 5 \text{ years} = 600 \text{ PY}

. Group 2 (disease at 24 months):

30 \text{ workers} \times 2 \text{ years} = 60 \text{ PY}

. Group 3 (disease at 48 months):

30 \text{ workers} \times 4 \text{ years} = 120 \text{ PY}

. Group 4 (loss to follow-up at 42 months):

120 \text{ workers} \times 3.5 \text{ years} = 420 \text{ PY}

. Total =

600 + 60 + 120 + 420 = 1,200 \text{ PY}

Incidence rate (incidence density) requires accounting for the exact duration of time each person remained disease-free and under observation.

Divide the incident cases by the total person-years and scale to

1,000

person-years.

\frac{60 \text{ cases}}{1,200 \text{ PY}} = 0.05 \text{ cases per PY}

. Multiplying by

1,000

yields

50.0

cases per

1,000

person-years.

Standardizing the rate to a standard population multiplier allows for comparable epidemiological reporting.

Key Concept

Incidence rate (incidence density) is calculated by dividing the number of new cases of a disease during a specific period by the total person-time contributed by the at-risk population during that period.

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