Is metformin (biguanide) safe for a patient with impaired renal function, specifically an estimated Glomerular Filtration Rate (eGFR) of 44?

Metformin Use in Patients with eGFR of 44 ml/min/1.73 m²

Metformin can be safely used in patients with an eGFR of 44 ml/min/1.73 m², but the dose should be reduced to 1000 mg daily to minimize the risk of lactic acidosis. 1

Dosing Recommendations for eGFR 30-44 ml/min/1.73 m²

• Metformin is recommended for patients with type 2 diabetes and chronic kidney disease (CKD) with eGFR ≥30 ml/min/1.73 m² 1
• For patients with eGFR between 30-44 ml/min/1.73 m², the dose should be reduced to 1000 mg daily (half the maximum dose) 1
• More frequent monitoring of renal function is required - every 3-6 months rather than annually 1
• FDA labeling indicates that metformin should not be initiated in patients with eGFR between 30-45 ml/min/1.73 m², but can be continued with dose reduction if already on therapy 2

Safety Considerations

• The risk of metformin-associated lactic acidosis is low in patients with mild to moderate renal impairment 3, 4
• Most episodes of metformin-associated lactic acidosis occur during acute illness when acute kidney injury contributes to reduced metformin clearance 1
• Temporary discontinuation of metformin is recommended during:
  • Serious intercurrent illness that increases risk of acute kidney injury 1
  • Procedures involving iodinated contrast agents 2
  • Surgery, angiography, or when patients are unwell at home ("sick-day rules") 1
  • Conditions with hypoxia, sepsis, or liver failure 1

Monitoring Requirements

• Monitor eGFR every 3-6 months when eGFR is <60 ml/min/1.73 m² 1
• Monitor for vitamin B12 deficiency in patients on long-term metformin therapy (>4 years) 1
• Watch for signs of lactic acidosis, particularly during acute illness 1

Benefits of Continuing Metformin

• Metformin remains the foundation for glycemic control in type 2 diabetes due to its safety, effectiveness, and affordability 1
• Evidence suggests potential cardiovascular benefits even in patients with reduced renal function 3, 5
• Observational studies show reduced mortality risk in patients with eGFR 30-45 ml/min/1.73 m² compared to other glucose-lowering therapies 1

Alternative Considerations

• If metformin alone is insufficient for glycemic control, consider adding an SGLT2 inhibitor if eGFR ≥20 ml/min/1.73 m² 1
• For patients unable to use metformin or SGLT2 inhibitors, GLP-1 receptor agonists are generally preferred as additional therapy 1
• Using estimated GFR rather than serum creatinine alone for determining metformin eligibility prevents unnecessary under-prescription 6

Common Pitfalls to Avoid

• Failing to reduce the metformin dose when eGFR falls below 45 ml/min/1.73 m² 1
• Not discontinuing metformin during acute illness or procedures that may cause acute kidney injury 1
• Using serum creatinine alone rather than eGFR to determine metformin eligibility 6
• Continuing metformin if eGFR falls below 30 ml/min/1.73 m² 1, 2
• Not accounting for patient's body surface area when calculating absolute clearance for drug dosing in patients significantly larger or smaller than average 1