What is the recommended dosage of metformin (Glucophage) for a patient with impaired renal function, specifically an estimated Glomerular Filtration Rate (eGFR) of 43?

Metformin Dosing for a Patient with eGFR 43 mL/min/1.73 m²

For a patient with an eGFR of 43 mL/min/1.73 m², metformin should be continued but the dose should be reduced to half the maximum recommended dose (typically 1000 mg daily total).

Dosing Recommendations

• For patients with eGFR 30-44 mL/min/1.73 m², the dose of metformin should be halved to 1000 mg daily total 1
• Immediate-release metformin can be initiated at 500 mg once daily and titrated slowly, not exceeding half the maximum recommended dose 1
• Extended-release metformin can be initiated at 500 mg daily and titrated slowly, not exceeding half the maximum recommended dose 1

Monitoring Requirements

• Renal function (eGFR) should be monitored more frequently when eGFR is <60 mL/min/1.73 m², specifically every 3-6 months 1, 2
• Monitor for signs and symptoms of lactic acidosis, which although rare, has a high fatality rate 3
• Monitor vitamin B12 levels if the patient has been on metformin for more than 4 years 1, 2

Safety Considerations

• Metformin is contraindicated when eGFR falls below 30 mL/min/1.73 m² due to increased risk of metformin-associated lactic acidosis 1, 4, 5
• Implement "sick day rules" - temporarily discontinue metformin during serious intercurrent illness, procedures requiring iodinated contrast, or situations that may compromise renal function 2, 4
• 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 4
• Re-evaluate eGFR 48 hours after any imaging procedure with contrast; restart metformin only if renal function is stable 4

Clinical Evidence

• The 2022 KDIGO guidelines strongly recommend metformin for patients with type 2 diabetes, CKD, and eGFR ≥30 mL/min/1.73 m² (Grade 1B recommendation) 1
• Recent large-scale cohort studies provide supporting evidence for the safe use of metformin in mild to moderate renal impairment (eGFR 30-60 mL/min/1.73 m²) with appropriate dose adjustments 5
• Some studies suggest metformin may have adverse effects on renal function in patients with moderate CKD, emphasizing the importance of close monitoring 6

Alternative Medications

• If glycemic targets are not met with the reduced metformin dose, consider adding a GLP-1 receptor agonist, which has documented cardiovascular benefits 1, 2
• SGLT2 inhibitors are recommended for most patients with type 2 diabetes and CKD with eGFR ≥20 mL/min/1.73 m², independent of HbA1c 1
• For patients who cannot tolerate metformin or if eGFR declines below 30 mL/min/1.73 m², alternative medications include DPP-4 inhibitors, insulin, or GLP-1 receptor agonists 1

Common Pitfalls to Avoid

• Failing to reduce the metformin dose when eGFR is 30-44 mL/min/1.73 m², which increases the risk of drug accumulation 1
• Not monitoring renal function frequently enough in patients with eGFR <60 mL/min/1.73 m² 1, 2
• Continuing metformin during acute illness or procedures that may cause acute kidney injury 4
• Overlooking the need to monitor vitamin B12 levels in long-term metformin users 1, 2