Kerions on a Man's Upper Lip

The risk of complications (such as lactic acidosis) that can be associated with metformin increases in patients with renal insufficiency. At what creatinine clearance should this agent be discontinued?

---- Ivanka Vassileva, MD Lawton, Okla

Currently, the contraindication for metformin use in patients with renal insufficiency is rigidly defined as a serum creatinine level of 1.5 mg/dL or higher in men and 1.4 mg/dL or higher in women. As you know, the serum creatinine level is often not a precise indicator of renal function. Measuring creatinine clearance, which is most often done using the Modification of Diet in Renal Disease (MDRD) equation, is probably a better way to assess for renal insufficiency. Current prescribing guidelines suggest confirming that creatinine clearance is normal in older persons and in others in whom the serum creatinine level may be misleading.

Metformin appears to accumulate once the creatinine clearance falls below 60 mL/min. Thus, it would be unwise to use metformin in any patient whose creatinine clearance is lower than this.

---- Silvio Inzucchi, MD Professor of Medicine Division of Endocrinology Yale University School of Medicine New Haven, Conn