Use of Metformin in the Setting of Mild-to-Moderate Renal Insufficiency

A common clinical conundrum faces all U.S. practitioners treating patients with type 2 diabetes. Today’s U.S. Food and Drug Administration prescribing guidelines for metformin contraindicate its use in men and women with serum creatinine concentrations ≥1.5 and ≥1.4 mg/dL (≥132 and ≥123 µmol/L), respectively. In a patient tolerating and controlled with this medication, should it automatically be discontinued as the creatinine rises beyond these cut points over time? Stopping metformin often results in poorly controlled glycemia and/or the need for other agents with their own adverse-effect profiles. Moreover, is the now widespread use of estimated glomerular filtration rate (eGFR) in lieu of serum creatinine levels creating even more confusion, especially in those with abnormalities in one but not the other indirect measure of renal function? Indeed, more than a decade and a half after metformin became available in the U.S., debate continues over the best approach in these settings (1–3). How many patients are unable to receive this medication on the basis of guidelines which, although well intentioned, are somewhat arbitrary and outdated based on modern assessments of renal status? There is some evidence that early treatment with metformin is associated with reduced cardiovascular morbidity and total mortality in newly diagnosed type 2 diabetic patients (4). However, the data come from a small subgroup of a much larger trial. In contrast, despite multiple trials of intensive glucose control using a variety of glucose-lowering strategies, there is a paucity of data to support specific advantages with other agents on cardiovascular outcomes (5–7). In the original UK Prospective Diabetes Study (UKPDS), 342 overweight patients with newly diagnosed diabetes were randomly assigned to metformin therapy (8). After a median period of 10 years, this subgroup experienced a 39% ( P = 0.010) risk reduction for myocardial infarction and a …