Assessment of warfarin requirements in post-bariatric surgery patients

Purpose: To quantify the change in weekly warfarin requirement following bariatric surgery in patients requiring chronic anticoagulation therapy.

Methods: This was a retrospective matched cohort study of patients chronically anticoagulated who underwent surgery between January 1, 1996 and December 31, 2010.  Bariatric surgery patients were matched to patients who underwent other abdominal procedures by date of surgery (± 2 yrs), age (± 5 yrs), and target INR range.   The primary study endpoint was change in weekly warfarin requirement from baseline assessed post-operatively at weeks 1 through 8 and months 3 and 6.

Results: A total of 86 patients were included - 27 (31%) in the bariatric surgery group and 59 (69%) in the control group. The bariatric patients had a statistically significant decrease in weekly warfarin requirement at all time points (week 1, p<0.05; all others, p<0.001), except 6 months (p>0.05). No statistically significant decreases in warfarin requirement were detected at any time in the control group (all p>0.05). Twenty patients (74.1%) in the bariatric surgery group experienced a ≥ 20% decrease in weekly warfarin requirement compared to 19 patients (32.2%, p = 0.004) in the control group. There were no differences in warfarin-related adverse events between groups (p>0.05).  

Conclusion: Weekly warfarin requirement dropped immediately following bariatric surgery, but returned to baseline after approximately 6 months.  This pattern was absent in a control group undergoing other types of abdominal surgery. Without vigilant monitoring and warfarin dosing adjustment, anticoagulation control in patients following bariatric surgery is likely to result in over anticoagulation. Future research should attempt to develop and validate a post-bariatric surgery warfarin dosing algorithm.