In a population of 50,000 people, 500 new cases of tuberculosis were diagnosed over one year. During the same year, the total number of people living with tuberculosis (both new and old cases) was 2,000. What is the incidence rate of tuberculosis per 1,000 population per year?

Explanation

## Calculation of Incidence Rate **Key Point:** Incidence rate is calculated using only **new cases** that occur during a defined period, divided by the **population at risk** during that same period. ### Step-by-Step Calculation 1. **Identify the numerator:** New cases of TB in one year = **500 cases** 2. **Identify the denominator:** Population at risk = **50,000 people** 3. **Apply the formula:** $$\text{Incidence Rate} = \frac{\text{New cases}}{\text{Population at risk}} = \frac{500}{50,000} = 0.01$$ 4. **Convert to per 1,000 population:** $$0.01 \times 1,000 = 10 \text{ per 1,000 per year}$$ ### Why Total Cases (2,000) Are Not Used **High-Yield:** The total number of people living with TB (2,000) represents **prevalence**, not incidence. Incidence specifically measures **new cases only**. The prevalence figure is a distractor that tests understanding of the distinction between these two measures. ### Interpretation **Clinical Pearl:** An incidence rate of 10 per 1,000 per year means that in this population, 10 new cases of TB develop for every 1,000 people at risk each year. This is useful for assessing the **force of disease** and evaluating the effectiveness of prevention programs. **Mnemonic:** **I = NEW / AT-RISK** — Incidence equals NEW cases divided by population AT-RISK.