New Metformin Rule 'Makes Me Incredibly Happy'

Today's topic is the new prescribing guidelines for metformin, just released from the US Food and Drug Administration (FDA). The headlines are as follows: You can use metformin in anyone whose estimated glomerular filtration rate (eGFR) is > 30 mL/minute/1.73 m² and you do not have to stop metformin in someone undergoing a dye study unless their EGFR is < 60 mL/minute/1.73 m².

Let me explain. We used to use serum creatinine cut-points to determine when we should prescribe metformin in patients with any degree of renal insufficiency. Now the FDA has done away with that guideline and really expanded the number of patients that we can safely keep on metformin. These are the rules:

• Test the eGFR in any patient before you start metformin. If it's > 45 mL/minute/1.73 m², you are fine. That patient is fully eligible to be on metformin.

• For the most part, the FDA does not recommend starting metformin in patients with an eGFR between 30 and 45 mL/minute/1.73 m². But they still consider metformin safe if your patient is on metformin already and seems to be deriving some benefit. So, patients down to an eGFR of 30 mL/minute/1.73 m² can remain on their metformin.