A prospective cohort study enrolled 2,000 individuals without diabetes. After 10 years of follow-up, 400 individuals developed type 2 diabetes. Among those who developed diabetes, 320 had a baseline BMI ≥30 kg/m², while among those who did not develop diabetes, 200 had a baseline BMI ≥30 kg/m². What is the odds ratio of developing type 2 diabetes in individuals with BMI ≥30 compared to those with BMI <30?

Explanation

## Calculation of Odds Ratio from Cohort Study Data **Step 1: Construct the 2×2 contingency table** | | Diabetes (Yes) | Diabetes (No) | Total | |---|---|---|---| | BMI ≥30 | 320 | 200 | 520 | | BMI <30 | 80 | 1,400 | 1,480 | | Total | 400 | 1,600 | 2,000 | **Derivation:** - Diabetes with BMI ≥30: 320 (given) - Diabetes without BMI ≥30: 400 − 320 = 80 - No diabetes with BMI ≥30: 200 (given) - No diabetes with BMI <30: 1,600 − 200 = 1,400 **Step 2: Apply the Odds Ratio formula** Odds Ratio (OR) = (a × d) / (b × c) Where: - a = Disease present, Exposure present = 320 - b = Disease present, Exposure absent = 80 - c = Disease absent, Exposure present = 200 - d = Disease absent, Exposure absent = 1,400 OR = (320 × 1,400) / (80 × 200) OR = 448,000 / 16,000 OR = **2.67** **Key Point:** Although this is a cohort study, the odds ratio can still be calculated and approximates the relative risk when the disease is rare. In this case, the disease is not rare (20% incidence), so OR and RR differ, but the question specifically asks for OR. **Clinical Pearl:** An OR of 2.67 indicates that individuals with BMI ≥30 have 2.67 times the odds of developing type 2 diabetes compared to those with BMI <30.