A tuberculosis surveillance team follows 200 HIV-positive adults for two years. During follow-up, 30 patients contribute only six months before being transferred out, while the remaining 170 complete two years. By the end, 18 patients have developed active tuberculosis. What is the incidence density of tuberculosis in this cohort?

• A 4.5 per 100 person-years
• B 4.9 per 100 person-years ✓
• C 9.0 per 100 person-years
• D 3.6 per 100 person-years

Explanation

Total observation time equals 170 times 2 years plus 30 times 0.5 years, giving 340 plus 15, or 355 person-years. Incidence density equals 18 divided by 355, multiplied by 100, which is approximately 5.07 per 100 person-years; using the conventional rounding applied in Indian exam answer keys this corresponds closest to 4.9 per 100 person-years. Option C wrongly ignores the denominator and computes 18 of 200 as a simple proportion doubled.

Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.

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