How high can creatinine go before you must stop or alter the metformin dose?
—MICHAEL QUIRK, PA-C, Cudjoe Key, Fla.

A principal concern in patients using metformin is the development of lactic acidosis. While uncommon, this side effect has a high fatality rate. Therefore, metformin should not be used in patients with predisposing factors (primarily impaired kidney function), defined as a serum creatinine concentration >1.4 mg/dL [124 mmol/L] in women and >1.5 mg/dL [133 mmol/L] in men. This correlates to an estimated glomerular filtration rate <60 mL/min/1.73 m2 in the elderly or in patients with reduced muscle mass (in whom serum creatinine does not accurately reflect renal function). For further discussion, see McCulloch D. Metformin in the treatment of diabetes mellitus. In: Rose BD, ed. UpToDate. Wellesley, Mass.: UpToDate; 2008.
—Reuben W. Zimmerman, PA-C (124-14)