Vitamin K antagonists (VKAs)
VKAs such as acenocoumarol, fenprocoumon, and warfarin, inhibit the carboxylation of the vitamin K-dependent coagulation factors II, VII, IX, and X in the liver necessary for coagulation and thus indirectly inhibit the coagulation process. VKAs are used for the prophylaxis and treatment of VTE and stroke prevention in atrial fibrillation. As VKAs are absorbed in the proximal intestine, bariatric surgery could have an impact on the absorption of VKAs.
Several studies investigated the effect of bariatric surgery on the daily dose of warfarin
121–124. All studies demonstrated that the warfarin dose was significantly reduced after RYGB surgery, especially in the six month postoperatively. After the direct postoperative phase, the required dose tended to gradually go back up to pre-surgical levels at six months to one year after surgery 121–124. It is not yet entirely clear why a lower dose is needed shortly after the operation. While it can be hypothesized that absorption may be increased because of crushing the tablets by patients directly after surgery, another explanation may lie in the changes that are associated with bariatric surgery that are to date not fully understood, such as metabolic changes 4. Also changes in diet during the first months after surgery may play a role. Similar to the results of platelet aggregation inhibitors, it seems that bariatric surgery may improve the response on VKAs compared to obese subjects, especially in the first months after surgery 121,122. It is known that compared to normal weight patients, obese patients require a higher average daily dose and also require more time to achieve therapeutic international normalized ratio​ (INR) 125. While no studies have been conducted on acenocoumarol and fenprocoumon after bariatric surgery, it may seem that these results also apply to these drugs. Overall, more frequent monitoring of the INR seems appropriate in the first year after bariatric surgery.