Should Metformin (biguanide oral hypoglycemic agent) be stopped prior to abdominal surgery?

Should Metformin Be Stopped Prior to Abdominal Surgery?

Yes, metformin should be stopped the night before elective abdominal surgery and should not be restarted until 48 hours postoperatively after confirming adequate renal function (eGFR ≥60 mL/min/1.73m²). 1, 2

Rationale for Discontinuation

The primary concern is metformin-associated lactic acidosis (MALA), a rare but life-threatening complication with mortality rates of 30-50%. 3, 1 While the incidence is low (2-9 cases per 100,000 patients/year), abdominal surgery creates multiple risk factors that substantially increase this risk. 3

Why Surgery Increases MALA Risk

Abdominal surgery specifically creates conditions that predispose to lactic acidosis:

• Hemodynamic instability and tissue hypoperfusion during major abdominal procedures can impair lactate clearance and cause metformin accumulation 3, 2
• Perioperative fasting and fluid restriction lead to dehydration and reduced renal perfusion 2
• Acute kidney injury is common postoperatively, even when not immediately apparent, reducing metformin clearance (metformin is 90% renally excreted) 2
• Potential bowel ischemia or mesenteric hypoperfusion during abdominal surgery generates lactate while metformin impairs hepatic lactate clearance 3, 4

Timing of Discontinuation

Stop metformin the night before surgery (approximately 12-24 hours preoperatively) for patients with normal renal function. 1, 5 The plasma half-life of metformin is 6.2 hours, so stopping the night before allows for significant drug clearance. 1

For Emergency Surgery

If emergency abdominal surgery is required and the patient has taken metformin recently:

• Proceed with surgery without delay—surgical source control takes priority 1
• Monitor lactate levels and renal function closely intraoperatively and postoperatively 1
• Ensure adequate fluid resuscitation with balanced crystalloids (lactated Ringer's or Plasma-Lyte, not normal saline which worsens acidosis) 6
• Be prepared for hemodialysis if MALA develops (metformin is dialyzable with clearance up to 170 mL/min) 2

When to Restart Metformin

Wait 48 hours after surgery before restarting metformin, and only after confirming:

• eGFR ≥60 mL/min/1.73m² (check renal function before restarting) 1, 2
• Patient is eating and drinking normally 1
• No ongoing hemodynamic instability (no vasopressor requirement) 1
• No evidence of sepsis, acute heart failure, or respiratory insufficiency 1, 2

The 48-hour waiting period is critical because surgery can cause transient renal impairment that may not be immediately apparent, and perioperative hemodynamic instability increases metformin accumulation risk. 1

High-Risk Patients Requiring Extra Caution

Certain patients have substantially elevated risk for MALA and require more aggressive management:

• Baseline renal impairment (eGFR 30-60 mL/min/1.73m²): Stop metformin 48-72 hours before surgery if possible 3, 2
• Elderly patients (≥65 years): Higher likelihood of hepatic, renal, or cardiac impairment 2
• Congestive heart failure: Impaired tissue perfusion and lactate clearance 3, 2
• Liver disease: Reduced hepatic lactate metabolism 3, 2
• Patients on ACE inhibitors, ARBs, or diuretics: Increased risk of perioperative acute kidney injury 3, 4

Evidence Regarding Continuation vs. Discontinuation

While one randomized trial found that continuing metformin perioperatively did not improve glucose control or raise lactate to clinically significant levels in elective non-cardiac surgery 7, this evidence is outweighed by:

• Multiple case reports of fatal MALA after abdominal surgery in patients who continued metformin 4, 8
• FDA labeling explicitly recommends discontinuation for surgical procedures where food/fluid intake is restricted 2
• Consensus guideline recommendations from the American College of Cardiology, American Heart Association, American Society of Anesthesiologists, and European Society of Anesthesiology all recommend stopping metformin perioperatively 1, 5

The 2024 AHA/ACC guidelines acknowledge that recent data suggest metformin may not be as strongly associated with lactic acidosis as previously feared, but the conservative approach of holding it perioperatively remains standard practice given the catastrophic consequences when MALA does occur. 1

Practical Algorithm for Abdominal Surgery

Preoperatively:

1. Stop metformin the night before elective surgery 1, 5
2. Check baseline eGFR 2
3. Ensure adequate hydration status 2
4. For emergency surgery, proceed without delay but alert team to metformin use 1

Intraoperatively:

• Use balanced crystalloids for fluid resuscitation 6
• Monitor for signs of tissue hypoperfusion 6
• Maintain mean arterial pressure ≥65 mmHg 6

Postoperatively:

• Check renal function at 48 hours 1, 2
• Restart metformin only if eGFR ≥60 mL/min/1.73m², patient eating/drinking normally, and hemodynamically stable 1
• If patient develops unexplained acidosis, check lactate immediately and consider MALA 2, 4

Common Pitfalls to Avoid

• Do not restart metformin at 48 hours if the patient has ongoing dehydration, vasopressor requirement, or any signs of organ hypoperfusion, even if renal function appears normal 1
• Do not use normal saline for resuscitation in patients at risk for MALA—it worsens hyperchloremic acidosis 6
• Do not delay surgical intervention in emergency cases to allow metformin clearance—surgical source control is paramount 1, 6
• Do not forget to document metformin discontinuation and communicate the 48-hour restart plan to the surgical team 2