Do I need to lower my metformin (biguanide) dose with an estimated Glomerular Filtration Rate (eGFR) of 30-39, indicating moderate to severe impaired renal function, while currently taking 1000mg?

Metformin Dosing with eGFR 30-39 mL/min/1.73m²

Yes, you need to reduce your metformin dose from 1000mg to 500mg daily with an eGFR of 30-39 mL/min/1.73m², as this represents moderate to severe renal impairment. 1, 2

Metformin Dosing Based on Kidney Function

eGFR 30-44 mL/min/1.73m² (Your Current Range)

• Metformin can still be used but requires dose reduction to half the maximum recommended dose 1
• For patients in this range, the recommended dose is 500mg daily 3
• More frequent monitoring of kidney function is required (every 3-6 months) 1, 4
• The risk of metformin accumulation and associated lactic acidosis increases with declining kidney function 2

Safety Considerations

• Metformin is primarily eliminated by the kidneys, and reduced kidney function leads to drug accumulation 2
• With an eGFR of 30-39, you are at the lower end of the acceptable range for metformin use 1
• Metformin is contraindicated when eGFR falls below 30 mL/min/1.73m² 2
• The FDA drug label specifically states that metformin should be discontinued if eGFR falls below 30 mL/min/1.73m² 2

Monitoring Requirements

• More frequent monitoring of kidney function is essential with your level of renal impairment 1
• Check eGFR at least every 3-6 months 4
• Monitor for signs of vitamin B12 deficiency if you've been on metformin for more than 4 years 1
• Be alert for symptoms of lactic acidosis (malaise, myalgias, abdominal pain, respiratory distress, somnolence) 2

Special Precautions

• Temporarily discontinue metformin during acute illness, surgery, or procedures with contrast dye 2
• Implement "sick day rules" - stop metformin during serious intercurrent illness that increases risk of acute kidney injury 1
• Avoid excessive alcohol intake as it potentiates metformin's effect on lactate metabolism 2
• Be particularly cautious with concomitant medications that may further impair kidney function 2

Alternative Medication Options

• If glycemic targets aren't met with the reduced metformin dose, consider adding a GLP-1 receptor agonist 1
• For patients with eGFR <30 mL/min/1.73m² (if your kidney function declines further), alternative diabetes medications include DPP-4 inhibitors, insulin, sulfonylureas, TZDs, or alpha-glucosidase inhibitors 1

Remember that while metformin remains beneficial even with moderate kidney impairment, the dose must be reduced to minimize the risk of adverse effects while maintaining efficacy 5, 6.