How long should metformin be stopped before surgery?

Metformin Management Before Surgery

Metformin should be stopped the night before surgery and not restarted until 48 hours after surgery, after confirming adequate renal function. 1, 2

Rationale for Stopping Metformin Before Surgery

• Metformin is associated with a risk of lactic acidosis, which is a rare but serious complication with mortality rates of 30-50% 1, 3
• The primary concern is the development of lactic acidosis in the perioperative period, particularly in patients with compromised renal function 4
• The plasma half-life of metformin is 6.2 hours, which means stopping the night before surgery allows for significant clearance 5

Risk Factors for Metformin-Associated Lactic Acidosis

• Renal impairment (creatinine clearance <60 mL/min) is the primary risk factor 1, 6, 4
• Other risk factors include:
  • Administration of iodinated contrast agents 5
  • Dehydration, which can occur during surgical fasting 4
  • Severe heart failure (left ventricular ejection fraction <30%) 1, 4
  • Hepatic impairment 4

Timing of Metformin Discontinuation

• For patients with normal renal function: Stop metformin the night before surgery 1, 2
• For patients with renal impairment: Consider stopping metformin earlier (48-72 hours before) and monitoring renal function 4
• For emergency surgery: Proceed with surgery but monitor lactate levels and renal function closely 7

Perioperative Management Algorithm

1. Preoperative Assessment:

   • Check renal function (eGFR) before surgery 4
   • Identify patients at high risk for lactic acidosis (renal impairment, heart failure, etc.) 1, 4

2. Discontinuation Protocol:

   • Normal renal function: Stop metformin the night before surgery 1, 2
   • Impaired renal function (eGFR <60 mL/min): Consider earlier discontinuation 4

3. Monitoring During Surgery:

   • Monitor lactate levels in high-risk patients 8
   • Watch for signs of lactic acidosis: arterial pH <7.35, blood lactate >5 mmol/L 5, 4

4. Restarting Metformin:

   • Wait 48 hours after major surgery 1, 5
   • Confirm adequate renal function before restarting 2, 4
   • Ensure patient is hemodynamically stable and taking oral nutrition 7

Recent Evidence on Metformin Continuation

• A 2018 randomized controlled trial found that continuing metformin perioperatively did not significantly raise lactate levels to a clinically relevant degree 9
• A 2019 study of CABG patients showed no difference in lactate levels or incidence of lactic acidosis when metformin was continued until the night before surgery 8
• The 2024 AHA/ACC guideline notes that recent data suggest metformin is not associated with lactic acidosis as previously feared 7

Common Pitfalls and Caveats

• Automatic suspension of metformin may not be necessary in all patients; checking renal function after surgery and withholding metformin if renal function deteriorates might be an acceptable alternative 5
• Patients with normal renal function and no other risk factors have a very low risk of developing lactic acidosis 7, 9
• Recognize metformin-associated lactic acidosis early: arterial pH <7.35, blood lactate >5 mmol/L, and detectable plasma metformin concentration 5
• If metformin-associated lactic acidosis is suspected, prompt hemodialysis is recommended 4

Special Considerations for SGLT2 Inhibitors

• SGLT2 inhibitors should be discontinued 3-4 days before surgery due to risk of euglycemic diabetic ketoacidosis 7
• These medications should not be restarted until the patient is clinically stable and has resumed a normal diet 7