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Difficulty: HardMeasures of Disease Frequency

A multicenter gastroenterology consortium establishes a prospective registry to evaluate the risk of cholangiocarcinoma in patients with primary sclerosing cholangitis. On January 1, 2020, the team screens 258 patients and excludes 4 who already have evidence of malignancy. The remaining 254 disease-free patients are followed until December 31, 2024.

During this exact 5-year observation period, the following events are recorded among the enrolled cohort:
- 30 patients are diagnosed with cholangiocarcinoma exactly 24 months after enrollment.
- 50 patients are lost to follow-up exactly 36 months after enrollment, with no evidence of malignancy prior to that point.
- 20 patients die from unrelated causes exactly 12 months after enrollment, without developing the cancer.
- The remaining patients complete the full 5-year follow-up period without developing the disease.

Assuming no other patients enter or leave the study, what is the incidence rate of cholangiocarcinoma in this cohort, expressed as cases per 1,0001,000 person-years?

Answer: 30 cases

Answer

30
The incidence rate (incidence density) is the number of new cases (30) divided by the total person-time contributed by the population at risk. After excluding the 4 prevalent cases, the 254 at-risk patients contribute person-time based on their specific follow-up duration. The 30 diagnosed patients contribute 60 person-years (PY), the 50 lost to follow-up contribute 150 PY, the 20 deceased contribute 20 PY, and the remaining 154 patients contribute 770 PY. This totals 1,000 PY. The rate is therefore 30 cases per 1,000 person-years.

Step-by-Step Solution

1
Determine the population at risk at baseline.
258 screened - 4 prevalent cases = 254 patients at risk.
Patients who already have the disease cannot be counted in the population at risk for developing incident (new) cases.
2
Calculate the person-time contributed by patients who were censored or experienced the event.
Diagnosed: 30 pts × 2 yrs = 60 PY. Lost to follow-up: 50 pts × 3 yrs = 150 PY. Deceased: 20 pts × 1 yr = 20 PY.
In a dynamic cohort, patients only contribute person-time for the exact duration they are actively observed and at risk.
3
Determine the number of patients who completed the full 5-year observation period and calculate their person-time.
154 remaining patients × 5 yrs = 770 PY.
The total cohort of 254 minus the 100 patients accounted for in Step 2 leaves 154 patients who were followed for the entire 5 years.
4
Calculate the total person-years of observation for the entire cohort.
60 + 150 + 20 + 770 = 1,000 total person-years.
The incidence density denominator requires the sum of all individual person-time contributed by the cohort.
5
Calculate the final incidence rate per 1,000 person-years.
30 incident cases / 1,000 person-years.
Incidence rate is the number of new cases divided by the total person-time at risk. Converted to a base of 1,000 PY, this equals 30.

Key Concept

Incidence rate (incidence density) is calculated by dividing the number of new disease cases by the total person-time contributed by all individuals in the population at risk during the observation period.
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Topics

Measures of Disease Frequency

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