Which of the following is a contraindication for metformin use?

• A. eGFR 60 mL/min/1.73 m2
• B. eGFR 25 mL/min/1.73 m2
• C. Normal hepatic function
• D. No comorbid kidney disease

Answer: (B)

Metformin is cleared by the kidneys, so when renal function falls, the drug can accumulate and increase the risk of lactic acidosis. The threshold most often cited is an eGFR below 30 mL/min/1.73 m2, at which point metformin is contraindicated. An eGFR of 60 indicates preserved kidney function, so there’s no issue with metformin. Normal hepatic function isn’t a contraindication because metformin isn’t hepatically metabolized. No comorbid kidney disease means there isn’t a renal barrier to starting or continuing the drug. Therefore, an eGFR of 25 mL/min/1.73 m2 represents a contraindication. In practice, if eGFR is 30–45, metformin may be used with caution and dose adjustments, and the drug should be held during acute kidney injury or when contrast studies are used.