A cohort of 10,000 factory workers without tuberculosis was followed for 2 years. During this period, 80 workers developed active TB. In the same factory, a cross-sectional survey at the end of 2 years identified 150 workers with active TB. Which of the following statements correctly interprets these two measures?

Explanation

## Distinguishing Incidence and Prevalence in a Cohort Study ### Definitions in This Context **Key Point:** - **Incidence** = new cases developing during the follow-up period (80 workers) - **Prevalence** = all existing cases at a point in time (150 workers at end of 2 years) ### Calculation **Incidence rate:** $$\text{Incidence} = \frac{80 \text{ new cases}}{10,000 \text{ at-risk cohort}} \times 1000 = 8 \text{ per 1,000 per 2 years}$$ Or expressed per 10,000: **80 per 10,000 per 2 years** **Prevalence at end of 2 years:** $$\text{Prevalence} = \frac{150 \text{ existing cases}}{\text{Total population at that time}} \times 1000$$ Expressed per 10,000: **150 per 10,000** (at a single point in time) ### Why Prevalence > Incidence | Factor | Explanation | |--------|-------------| | **New cases** | 80 workers developed TB during 2 years | | **Existing cases** | 150 workers have TB at end of 2 years | | **Difference** | 70 cases (150 − 80) were already present at baseline or diagnosed earlier | | **Implication** | Prevalence includes both incident and prevalent cases; incidence counts only new cases | **High-Yield:** In the same population, prevalence ≥ incidence always, because prevalence includes all existing cases (old + new). **Mnemonic:** **I**ncidence = **I**nterval (new cases over time); **P**revalence = **P**ool (all cases at one moment). **Clinical Pearl:** If a disease has high incidence but low prevalence, it is either rapidly fatal or rapidly cured. If incidence is low but prevalence is high, the disease is chronic with long duration.