## Understanding Odds Ratio in Case-Control Studies **Key Point:** The odds ratio (OR) in a case-control study estimates the odds of *exposure* among cases versus the odds of *exposure* among controls. It does NOT directly estimate relative risk. ### Why OR ≠ RR in Case-Control Design In a case-control study: - You START with people who have the disease (cases) and those who don't (controls) - You look *backward* to determine who was exposed - You cannot directly calculate incidence or cumulative risk - The OR approximates RR only when the disease is rare (< 10% prevalence) ### Correct Interpretation of OR = 3.2 The odds ratio of 3.2 means: - Among women **with VTE** (cases), the odds of prior OCP use = 3.2 × (odds of OCP use among controls) - In plain language: women with VTE are 3.2 times as likely to have used OCPs compared to women without VTE - This is a *retrospective* comparison of exposure history ### Why the 95% CI Matters | Feature | Interpretation | |---------|----------------| | CI excludes 1.0 | Association is statistically significant (p < 0.05) | | Lower bound = 2.1 | Even the most conservative estimate shows > 2-fold increased odds | | Upper bound = 4.8 | The true OR could be as high as 4.8 | **High-Yield:** In case-control studies, always interpret OR as "odds of exposure among cases vs. controls," NOT "risk in exposed vs. unexposed." **Clinical Pearl:** The OR of 3.2 suggests a strong association between OCP use and VTE. In clinical practice, this finding would support counseling Indian women about VTE risk before prescribing OCPs, especially those with additional thrombotic risk factors. **Warning:** ~~"OR of 3.2 means 3.2 times higher risk"~~ — this is a common misinterpretation. The correct phrasing is "3.2 times higher *odds* of prior exposure."
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