How many days prior to surgery should metformin (biguanide oral hypoglycemic) be stopped?

Perioperative Management of Metformin

Metformin should be stopped the night before surgery and not restarted until 48 hours after major surgery, once adequate renal function has been confirmed. 1

Rationale for Metformin Discontinuation

Metformin discontinuation before surgery is primarily related to the risk of lactic acidosis, a rare but potentially fatal complication with mortality rates of 30-50%. This risk increases in certain perioperative conditions:

• Renal function impairment (creatinine clearance <60 mL/min)
• Administration of iodinated contrast agents
• Situations that could alter renal function:
  • Dehydration
  • Fasting
  • Medications affecting kidney function (ACE inhibitors, ARBs, diuretics, NSAIDs)
• Severe heart failure (LVEF <30%)

Timing of Discontinuation

The timing of metformin discontinuation depends on the type of surgery and patient risk factors:

Standard Recommendation:

• Stop metformin the night before surgery 1
• This recommendation is supported by the 2018 Anaesthesia guidelines, which represent the most recent high-quality evidence on this topic

Special Circumstances:

• For patients with known renal failure: Stop metformin 48 hours before the procedure 2
• For patients undergoing procedures with IV contrast: Consider individual risk factors for contrast-induced nephropathy 2

Postoperative Management

• Do not restart metformin before 48 hours for major surgery 1
• Confirm adequate renal function before restarting 1
• For minor/ambulatory surgery: May restart earlier if renal function is stable and patient has resumed normal oral intake

Evidence Analysis

Recent research has challenged traditional concerns about metformin and lactic acidosis:

• A 2018 randomized controlled trial found that continuation of metformin during elective non-cardiac surgery did not raise lactate levels to a clinically relevant degree 3
• A 2019 study of CABG patients showed no significant differences in lactate levels when metformin was continued until the night before surgery 4

However, despite these findings, the most recent guidelines from Anaesthesia (2018) still recommend discontinuing metformin the night before surgery 1. This conservative approach prioritizes patient safety given the potential severity of lactic acidosis, even if rare.

Common Pitfalls and Caveats

• Failure to assess renal function: Always check renal function before restarting metformin
• Overlooking risk factors: Patients with multiple comorbidities (renal impairment, heart failure) require special attention
• Premature restart: Restarting metformin too early after surgery, especially before confirming stable renal function
• Inconsistent guidelines: Different guidelines may recommend different timing for discontinuation (night before vs. 48 hours before)

Algorithm for Metformin Management in Surgery

1. Preoperative Assessment:

   • Check renal function (eGFR or creatinine clearance)
   • Identify risk factors for lactic acidosis

2. Discontinuation Timing:

   • Standard patient: Stop metformin the night before surgery
   • High-risk patient (renal dysfunction, contrast procedure): Stop 48 hours before

3. Postoperative Management:

   • Monitor renal function
   • Restart metformin only after:
     • 48 hours post-major surgery
     • Confirmation of stable renal function
     • Resumption of normal oral intake

By following these evidence-based recommendations, clinicians can minimize the risk of metformin-associated lactic acidosis while maintaining appropriate glycemic control in the perioperative period.