A case-control study investigating the association between oral contraceptive use and venous thromboembolism (VTE) enrolled 500 women with VTE and 500 age-matched controls without VTE. Among the VTE cases, 150 reported current oral contraceptive use, while 350 did not. Among the controls, 50 reported current oral contraceptive use, while 450 did not. Calculate the odds ratio and interpret its clinical significance in the context of risk estimation.

Explanation

## Calculation of Odds Ratio **Formula:** OR = (a × d) / (b × c) Where: - a = VTE cases exposed (OCP use) = 150 - b = VTE cases unexposed (no OCP) = 350 - c = Controls exposed (OCP use) = 50 - d = Controls unexposed (no OCP) = 450 **OR = (150 × 450) / (350 × 50) = 67,500 / 17,500 = 3.857 ≈ 4.29** ## Interpretation **Key Point:** An OR of 4.29 means women using oral contraceptives have 4.29 times the odds of VTE compared to non-users. **Percentage increase in odds:** (OR − 1) × 100 = (4.29 − 1) × 100 = **329%** ## Clinical Significance **High-Yield:** This finding aligns with established epidemiological evidence that oral contraceptives (especially third-generation formulations) increase VTE risk 3–4 fold. This is why: - Detailed thrombotic risk assessment is mandatory before OCP prescription - Personal or family history of VTE is a contraindication - Alternative contraception is preferred in high-risk women **Important Distinction:** The OR approximates **relative risk (RR)** only when the disease is rare in the population. Since VTE is relatively uncommon (~1–2 per 1000 per year), the OR ≈ RR here, making the 4.29 figure clinically meaningful for risk communication.