A justification for less restrictive guidelines on the use of metformin in stable chronic renal failure.

AIM: The aim was to justify less restrictive use of metformin in stable chronic renal failure, because a literature review reveals metformin is associated with a significantly lower incidence of cardiovascular events and mortality compared with other hypoglycaemic agents, and metformin-associated lactic acidosis is rare and causation uncertain. Studies on intentional metformin overdose and metformin bioavailability, renal clearance and plasma metformin in renal impairment provide evidence in support of a less restrictive use of metformin.
METHODS: In metformin overdose (n = 22), lactic acidosis was not inevitable with a plasma metformin > 40 mg/l (therapeutic level c. 1 mg/l): Severe lactic acidosis (pH ≤ 7.21, plasma lactate ≥ 11 mmol/l, n = 8) did not occur unless plasma metformin was > 40 mg/l. Plasma lactate was a more consistent predictor of pH than plasma metformin, with plasma lactate ≤ 4.7 being associated with a pH ≥ 7.34. A likely 'safe' plasma lactate is < 3.5 mmol/l and plasma metformin < 10 mg/l.
RESULTS: Plasma metformin can be predicted from estimated glomerular filtration rate and metformin dose. Reported plasma metformin in renal failure was always less than predicted plasma metformin. Predicted plasma metformin (mg/l), with an estimated glomerular filtration rate of 30 ml/min and metformin 2000 mg/day was 6.8; an estimated glomerular filtration rate of 20 ml/min and metformin 1500 mg/day was 5.1; an estimated glomerular filtration rate of 10 ml/min and metformin 500 mg/day was 4.4.
CONCLUSION: Metformin accumulates in renal failure and, although accumulation does not always lead to lactic acidosis, dose modification to achieve a predicted plasma metformin < 10 mg/l is suggested. As plasma metformin is not routinely available, plasma lactate should be useful in monitoring the use of metformin in renal failure.