A cohort of 200 patients with chronic hepatitis B is followed for 5 years to detect development of hepatocellular carcinoma. Five patients develop the tumour, one each at the end of years 1, 2, 3, 4 and 5; the remaining 195 patients complete the full 5 years without disease. What is the incidence density of hepatocellular carcinoma?

• A 2.5 per 1000 person-years
• B 5.0 per 1000 person-years
• C 5.1 per 1000 person-years ✓
• D 25.0 per 1000 person-years

Explanation

Person-years contributed equal (195 x 5) plus the follow-up times of the five cases (1+2+3+4+5 = 15), giving 990 person-years. The incidence density is 5 divided by 990, which is about 5.05 per 1000 person-years. The option of 5.0 per 1000 ignores the partial person-time contributed by cases before diagnosis, while 25.0 per 1000 wrongly treats the denominator as 200 persons.

Reference: Gordis Epidemiology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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