Comparison of Warfarin Dosages and International Normalized Ratios Before and After Roux-en-Y Gastric Bypass Surgery

Pharmacotherapy. 2015 Sep;35(9):876-80. doi: 10.1002/phar.1632.


Study objective: As data are limited for dosing warfarin in patients who have undergone bariatric surgery, our objective was to gather data on warfarin pharmacotherapy to aid in the prescribing practices and dosage adjustment of warfarin after Roux-en-Y gastric bypass (RYGB) surgery.

Design: Retrospective medical record review.

Setting: Anticoagulation clinic.

Patients: Twelve patients who underwent RYGB surgery between 2009 and 2011.

Measurements and main results: Data were collected on patient demographics, therapeutic indications for warfarin, weekly warfarin doses, and international normalized ratio (INR) values before and after RYGB surgery. To examine whether the relationship between the warfarin dose and therapeutic effect (i.e., INR) of warfarin changed from before to after surgery, we generated a ratio between each INR measurement and the weekly warfarin dose that immediately preceded it (warfarin dose/INR ratio). Mean weekly warfarin dose after RYGB surgery decreased significantly relative to presurgery dose (estimated marginal mean ± SE presurgery dose 37.08 ± 3.31 mg vs postsurgery dose 28.08 ± 3.27 mg; [F(205.63) = 13.23, p<0.001]), whereas INRs increased significantly after surgery from presurgery levels (estimated marginal mean ± SE presurgery INR 2.36 ± 0.15 vs postsurgery INR 2.94 ± 0.14; [F(213.76) = 6.55, p<0.05]). The ratio of warfarin dose/INR from presurgery (estimated marginal mean ± SE 17.64 ± 1.67) to postsurgery (estimated marginal mean ± SE 10.94 ± 1.64) was also significantly reduced [F(206.47) = 20.61, p<0.001], suggesting that a lower warfarin dose was required to produce the desired INR after surgery compared with before surgery.

Conclusion: Warfarin dosages tended to decrease after RYGB surgery, in agreement with previously published literature. These findings suggest the need for increased INR monitoring and patient education for patients taking warfarin after RYGB surgery.

Keywords: INR; Roux-en-Y gastric bypass; anticoagulant; bariatric surgery; international normalized ratio; pharmacokinetic; warfarin.

Publication types

  • Comparative Study

MeSH terms

  • Adult
  • Anticoagulants / administration & dosage*
  • Anticoagulants / pharmacology
  • Dose-Response Relationship, Drug
  • Female
  • Gastric Bypass / methods*
  • Humans
  • International Normalized Ratio
  • Male
  • Middle Aged
  • Retrospective Studies
  • Warfarin / administration & dosage*
  • Warfarin / pharmacology


  • Anticoagulants
  • Warfarin