Metformin Dosing at eGFR 45 mL/min/1.73m²
At an eGFR of 45 mL/min/1.73m², metformin can be continued at standard dosing up to 2000-2550 mg daily without mandatory dose reduction, though you should consider reducing the dose to half maximum (1000-1250 mg daily) if the patient has advanced age or concomitant liver disease. 1
Dosing Algorithm at eGFR 45
Your patient sits at a critical threshold where guidelines diverge slightly based on individual risk factors:
Standard Approach (Most Patients)
- Continue full-dose metformin up to 2000-2550 mg daily divided throughout the day 1
- The FDA label supports continuation without mandatory adjustment at this eGFR level 2
- This eGFR falls within the 45-59 mL/min/1.73m² range where standard dosing remains appropriate 1, 3
Risk-Based Dose Reduction (High-Risk Patients)
Consider reducing to 1000-1250 mg daily maximum if your patient has: 1, 3
- Advanced age (elderly patients)
- Concomitant liver disease
- Other risk factors for lactic acidosis
The Korean consensus specifically recommends not initiating metformin between eGFR 30-44, but your patient at 45 is just above this cutoff 4
Critical Monitoring Requirements
- Check eGFR every 3-6 months given the eGFR is below 60 mL/min/1.73m² 1, 3, 5
- Monitor vitamin B12 levels if metformin use exceeds 4 years 1, 3, 5
- Implement "sick day rules": temporarily stop metformin during acute illness that could precipitate acute kidney injury 3, 5
Safety Thresholds to Remember
The evidence is clear on these hard stops:
- eGFR 45-59: Standard dosing acceptable, consider reduction in high-risk patients 1, 3
- eGFR 30-44: Reduce dose to half maximum (typically 1000 mg daily) 1, 3, 4
- eGFR <30: Absolute contraindication—discontinue metformin completely 1, 3, 2, 4
Contrast Media Precautions
If your patient requires iodinated contrast imaging: 2
- Discontinue metformin at the time of or prior to the procedure
- Re-evaluate eGFR 48 hours after imaging
- Restart only if renal function remains stable
Clinical Context
The cardiovascular and mortality benefits of metformin likely outweigh the minimal lactic acidosis risk when dosed appropriately at this eGFR level 1. Research supports that metformin at appropriate doses in mild-to-moderate renal impairment (eGFR 30-60) is associated with lower cardiovascular events and mortality without increased lactic acidosis risk 6. A 2021 pharmacokinetic study demonstrated that doses up to 2250 mg daily maintain safety targets even in CKD stage 3a (eGFR 45-59) 7.
Common Pitfalls to Avoid
- Don't automatically reduce the dose at eGFR 45 unless risk factors are present—this may lead to suboptimal glycemic control 1
- Don't forget to reassess if eGFR drops below 45, as dose reduction becomes mandatory at eGFR 30-44 1, 3
- Don't continue metformin during acute illness without reassessing—implement sick day rules 3, 5, 8