An occupational health department at a petrochemical manufacturing plant is evaluating the… — USMLE Step 3

An occupational health department at a petrochemical manufacturing plant is evaluating the incidence of chronic irritant contact dermatitis (CICD) among its workers. On January 1, 2024, a total of $1,400$ employees undergo a baseline dermatological screening. The screening reveals that $200$ employees already have CICD. These individuals are provided treatment but remain employed. The remaining disease-free employees are enrolled in a $2$ -year prospective cohort study. The following timeline of events occurs:

Date	Event Description
January 1, 2024	Baseline screening (Study start)
July 1, 2024	$200$ disease-free employees resign and are lost to follow-up
January 1, 2025	$100$ employees are newly diagnosed with CICD
January 1, 2026	$50$ employees are newly diagnosed with CICD (Study end)

Assume no other losses to follow-up or deaths occur, and employees diagnosed with CICD remain employed but are no longer considered at risk for a new diagnosis. Based on these data, what is the incidence rate (incidence density) of CICD in this cohort over the $2$ -year study period?

A
$62.5$ per $1,000$ person-years
$75.0$ per $1,000$ person-yearsAnswer
C
$107.1$ per $1,000$ person-years
D
$125.0$ per $1,000$ person-years
E
$175.0$ per $1,000$ person-years

Answer

The incidence rate is

75.0

per

1,000

person-years.

Incidence rate (incidence density) is calculated by dividing the number of new cases by the total person-years at risk. First, prevalent cases (

200

) must be excluded from the screened population (

1,400

), leaving an initial at-risk cohort of

1,200

. Person-years (PY) are computed based on precise follow-up time:

200

employees followed for

0.5

years =

100

PY;

100

newly diagnosed employees followed for

1.0

year =

100

PY;

50

newly diagnosed employees followed for

2.0

years =

100

PY; and the remaining

850

disease-free employees followed for the full

2.0

years =

1,700

PY. Summing these yields a total of

2,000

person-years. The number of incident cases is

150

(

100

in 2025 +

50

in 2026). The incidence rate is therefore

\frac{150}{2,000} = 0.075

, which equals

75.0

per

1,000

person-years.

Step-by-Step Solution

Determine the initial population at risk.

1,400

total screened -

200

prevalent cases =

1,200

employees at risk.

Incidence measures only evaluate new cases among those at risk; individuals already possessing the disease must be excluded from the baseline denominator.

Calculate person-years (PY) for individuals who did not complete the full 2-year follow-up disease-free.

200

lost at

0.5

years =

100

PY.

100

diagnosed at

1.0

year =

100

PY.

50

diagnosed at

2.0

years =

100

PY.

In a dynamic cohort, individuals contribute time to the denominator only for the exact duration they are observed and disease-free.

Calculate person-years for individuals who completed the study disease-free.

1,200

initial -

200

(lost) -

150

(diagnosed) =

850

employees remaining.

850

employees ×

2.0

years =

1,700

PY.

The remaining employees were observed for the entirety of the 2-year study period without developing the condition.

Sum the total person-years and calculate the incidence rate.

Total PY =

100 + 100 + 100 + 1,700 = 2,000

PY. New cases =

150

. Incidence rate =

\frac{150}{2,000} = 0.075

, or

75

per

1,000

PY.

Incidence rate (density) is defined as the number of new cases divided by the total person-time at risk.

Key Concept

Incidence rate (incidence density) in a dynamic cohort requires excluding prevalent cases at baseline and calculating exact person-time at risk based on attrition and time of diagnosis.

Question

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Key Concept

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