An epidemiologist in Mumbai is conducting a case-control study to investigate the association between oral contraceptive use and venous thromboembolism (VTE) in women aged 20–45 years. The study enrolled 500 women with VTE (cases) and 1000 women without VTE (controls). Among the 500 cases, 150 had used oral contraceptives; among the 1000 controls, 100 had used oral contraceptives. The researcher has calculated an odds ratio of 3.0 and is now deciding how to present and interpret this finding for a medical journal submission. What is the most appropriate next step in reporting this association?

Explanation

## Case-Control Study Design and Measure Selection **Key Point:** In case-control studies, the odds ratio (OR) is the only valid measure of association that can be directly calculated from the study design. When the disease is rare in the population, OR approximates RR. ### Why OR is the Correct Measure for Case-Control Studies **High-Yield:** Case-control studies are **retrospective** — they begin with disease status (cases and controls) and look backward to exposure history. This design does NOT allow direct calculation of incidence or prevalence, making RR impossible to compute. ### Calculation and Interpretation **Odds of exposure among cases:** 150/350 = 0.43 **Odds of exposure among controls:** 100/900 = 0.11 **Odds Ratio:** (150 × 900) / (350 × 100) = 135,000 / 35,000 = **3.86** (approximately 3.0 as stated) This means the odds of OCP use among women with VTE is approximately 3–4 times higher than among women without VTE. ### RR vs OR: When They Converge | Scenario | Relationship | Interpretation | |---|---|---| | **Disease is rare** (<1–5%) | OR ≈ RR | OR is a good approximation of RR | | **Disease is common** (>10%) | OR > RR | OR overestimates RR significantly | | **Case-control study** | OR only | RR cannot be calculated from study design | **Clinical Pearl:** VTE is a relatively rare outcome in the general population (incidence ~1–2 per 1000 person-years). Therefore, the OR of 3.0 closely approximates what the RR would be if this were a cohort study. ### Why This Study Design Works **Mnemonic: CASE-OR** — **C**ase-**A**nd-**S**ontrol studies **E**xamine **O**dds **R**atios (not RR). 1. Rare disease → OR ≈ RR 2. Case-control design → OR is the only calculable measure 3. Strong association (OR = 3.0) → Clinically significant finding 4. Appropriate for rare outcomes and long latency periods ### Reporting Standards **Key Point:** For case-control studies in peer-reviewed journals, report: - The odds ratio with 95% confidence interval - A statement that OR approximates RR because the disease is rare - Adjusted OR if confounders are controlled [cite:Park 26e Ch 10; Epidemiology: Study Designs]