A case-control study of 500 patients with myocardial infarction (MI) and 500 age-matched controls without MI examined the association between high dietary salt intake and MI risk. Among MI cases, 320 had high salt intake; among controls, 180 had high salt intake. What is the odds ratio, and what does it represent in this study design?

Explanation

## Calculation and Interpretation of Odds Ratio in Case-Control Studies **Calculation:** - Odds of high salt intake among MI cases = 320 / (500 − 320) = 320 / 180 = 1.78 - Odds of high salt intake among controls = 180 / (500 − 180) = 180 / 320 = 0.5625 - **Odds Ratio (OR) = 1.78 / 0.5625 = 3.16 ≈ 3.2** (or using the cross-product formula: (320 × 320) / (180 × 180) = 3.16) *Note: The closest approximation in the options is 2.84, which may reflect a slight variation in calculation methodology or rounding; however, the conceptual answer remains the correct one.* **Key Point:** In a case-control study, the odds ratio is the **ratio of the odds of exposure among cases to the odds of exposure among controls**. It does NOT directly measure incidence or probability of disease in exposed individuals. **Clinical Pearl:** When the disease is rare (<10% in the population), the OR approximates the relative risk (RR). However, when disease prevalence is high (as in hospital-based case-control studies), the OR may overestimate the true RR. **High-Yield Fact:** The OR is the **only measure of association** that can be calculated directly from a case-control study design, because cases and controls are selected based on disease status, not exposure status. Therefore, incidence and relative risk cannot be directly computed.