Association of preoperative metformin use with postoperative mortality and morbidity in type 2 diabetes patients undergoing noncardiac surgery: a retrospective cohort study

BACKGROUND: Diabetes mellitus (DM) is prevalent among adults, many of whom require surgical interventions. Although metformin may improve postoperative outcomes by reducing inflammation, its effects on postoperative mortality and complications remain unclear. This study aimed to determine whether preoperative metformin use is associated with improved postoperative outcomes after noncardiac surgery. METHODS: This retrospective study included adult patients with type 2 DM who underwent noncardiac surgery between 2011 and 2019. Patients were assigned to one of two groups based on the use of preoperative metformin at admission. To evaluate dose-related effects, patients in the metformin group were further divided into low- and high-dose groups based on daily dose (< or ≥ 1,000 mg). The primary outcome was one-year mortality after surgery, and the secondary outcomes were 30-day mortality, five-year mortality, and postoperative complications in major organs within 7 d. RESULTS: Among 22 944 patients, 12 536 (54.6%) were exposed to preoperative metformin. After inverse probability of treatment weighting, preoperative metformin use was associated with a reduced one-year mortality (hazard ratio: 0.76, 95% CI [0.68-0.85]). For secondary outcomes, metformin use decreased postoperative complications in respiratory (odds ratio [OR]: 0.76, 95% CI [0.61-0.93]) and renal systems (OR: 0.66, 95% CI [0.58-0.74]). In a dose-related analysis, both doses were associated with a lower risk of postoperative mortality, with reductions in respiratory complications primarily due to high-dose metformin (OR: 0.69, 95% CI [0.54-0.89]). CONCLUSION: Preoperative use of metformin is associated with reduced postoperative mortality and complications in diabetic patients undergoing noncardiac surgery.