Rationale: Patients who are 6 to 18 months post-bariatric surgery generally have a 30-40% decrease in excess body weight. This decrease in body weight reduces volume of distribution of drug, potentially affecting Cmax. Results of the published Phase 1 apixaban study found that exposure to a dose of apixaban was inversely related to body weight. Therefore, the investigators should expect higher plasma levels of apixaban in patients who are 6 to 18 months post-bariatric surgery when compared to pre-op values.