In a case-control study investigating the association between oral contraceptive use and venous thromboembolism, researchers found that women with VTE were significantly more likely to report a family history of thrombophilia than controls. However, when the analysis was stratified by thrombophilia status, the association between OCP use and VTE disappeared in both strata. Which type of bias or confounding is most likely responsible for this finding?

Explanation

## Understanding the Scenario The study shows that: 1. OCP use appears associated with VTE overall 2. When stratified by thrombophilia status, the association disappears in both strata 3. Thrombophilia is more common in VTE cases ## Definition of Confounding **Key Point:** Confounding occurs when a third variable (the confounder) is: - Associated with the exposure (OCP use) - Associated with the outcome (VTE) - NOT on the causal pathway between exposure and outcome - Causes a spurious or distorted association ## Analysis of This Case Thrombophilia acts as a confounder because: - It is a risk factor for VTE (associated with outcome) - It may influence OCP use decisions or reporting (associated with exposure) - It is NOT caused by OCP use - When controlled for (stratified), the OCP-VTE association disappears **High-Yield:** When stratification eliminates an association, the stratifying variable is the confounder. This is the classic definition of confounding. ## Why This Is Not Other Types of Bias | Bias Type | Why Not This Case | |-----------|-------------------| | Selection bias | Affects who enters the study, not the relationship between variables | | Confounding by indication | Refers to confounding by the reason for treatment; here thrombophilia is a risk factor, not an indication | | Information bias | Affects accuracy of measurement, not the presence of a third variable | **Clinical Pearl:** In observational studies, thrombophilia screening status can confound the OCP-VTE relationship because women with known thrombophilia may avoid OCPs, while unscreened women use them freely.