The Question:

If a type 2 diabetic patient is on Metformin and receives iodinated contrast during SWL, Metformin should be withheld for at least 72 hours.

The Answer: False

According to the CDC, over 108 million Americans have diabetes or pre-diabetes.  Not surprisingly, many type 2 diabetics are also stone sufferers and will need the surgical management of their calculi.  Many of these patients will be on Metformin as a first line therapy.

While Metformin is safe and has very few side effects, it has been rarely associated with severe lactic acidosis, which can be fatal.  Because iodinated contrast can cause acute renal injury, the American College of Radiology in the past has recommended stopping Metformin before and after an IV contrast study because Metformin is eliminated through the kidneys and its accumulation could increase the risk of lactic acidosis.

The most recent ACR recommendations, however, have recognized that every case of Metformin lactic acidosis has occurred on patients that were inappropriately on Metformin.  As a consequence, current recommendations are that only patients with a eGFR < 30 ml/min/1.73m2 or evidence of acute kidney injury should have Metformin withheld for 48 hours after an iodinated contrast study or until kidney function “normalizes.”