Metformin Dosing in Patients with Impaired Renal Function
Metformin is contraindicated in patients with an estimated glomerular filtration rate (eGFR) below 30 mL/min/1.73 m², and requires specific dose adjustments for patients with reduced kidney function. 1
Dosing Recommendations Based on Renal Function
- eGFR ≥60 mL/min/1.73 m²: Standard dosing with no adjustments required (up to maximum 2550 mg daily) 2
- eGFR 45-59 mL/min/1.73 m²: Standard dosing can be continued, but consider dose reduction in patients with additional risk factors such as advanced age or concomitant liver disease 2, 3
- eGFR 30-44 mL/min/1.73 m²: Reduce to half of the maximum recommended dose (maximum 1000-1250 mg daily) 2, 4
- eGFR <30 mL/min/1.73 m²: Metformin is contraindicated 2, 1
Specific Dosing Guidelines by eGFR Range
- eGFR 15-29 mL/min/1.73 m²: Metformin should not be used (contraindicated) 1, 4
- eGFR 30-44 mL/min/1.73 m²: Maximum 1000 mg daily in divided doses 4
- eGFR 45-59 mL/min/1.73 m² (CKD Stage 3a): Maximum 1700 mg daily in divided doses 4
- eGFR ≥60 mL/min/1.73 m²: Up to standard maximum dose of 2550 mg daily 1, 4
Monitoring Requirements
- Assess renal function prior to initiation of metformin and periodically thereafter 1
- For patients with eGFR <60 mL/min/1.73 m², monitor renal function every 3-6 months 2, 3
- Monitor for vitamin B12 deficiency in patients on long-term metformin therapy (>4 years) 2, 3
Special Considerations
- Discontinue metformin at the time of, or prior to, iodinated contrast imaging procedures in patients with:
- eGFR between 30-60 mL/min/1.73 m²
- History of liver disease, alcoholism, or heart failure
- Scheduled for intra-arterial iodinated contrast 1
- Re-evaluate eGFR 48 hours after imaging procedure; restart metformin if renal function is stable 1
- Implement "sick day rules" to stop metformin during serious intercurrent illness that increases risk of acute kidney injury 2
Clinical Implications
- Using serum creatinine alone (rather than eGFR) for metformin eligibility may unnecessarily restrict its use in patients who could benefit 5
- The risk of lactic acidosis with metformin is low when dosed appropriately according to renal function 4, 6
- When eGFR falls below 45 mL/min/1.73 m², consider adding or switching to alternative diabetes medications suitable for reduced renal function 2, 3
Practical Approach
- Calculate eGFR using CKD-EPI equation (preferred over serum creatinine alone) 5, 6
- Select appropriate maximum daily dose based on eGFR category
- Start at lower doses and titrate up as tolerated
- Schedule appropriate monitoring of renal function based on eGFR level
- Educate patient about sick day rules and when to temporarily discontinue medication