Aims The reporting of estimated glomerular filtration rate (eGFR) will identify people with diabetes who have previously undiagnosed renal impairment. Our current guideline recommends discontinuation of metformin when serum creatinine > 150 µmol/l. We examined the implications of replacing this with a criterion based on eGFR.
Methods The Lothian diabetes register was used to identify patients with Type 2 diabetes for whom age, sex and creatinine measurements within the last 15 months were available. eGFR was calculated using the abbreviated Modification of Diet in Renal Disease (MDRD) equation.
Results Of 19 981 patients with Type 2 diabetes, 11 297 were taking metformin in accordance with our current guideline. Of these, 9259 (82.0%) had at least stage 2 renal impairment (eGFR < 90 ml/min per 1.73 m2) and 2880 (25.5%) had at least stage 3 renal impairment (eGFR < 60 ml/min per 1.73 m2). Changing to an eGFR threshold of 36 ml/min per 1.73 m2 would have a neutral effect on the number of patients eligible for metformin therapy and would permit its use in patients with creatinine concentrations as high as 179 µmol/l. An eGFR threshold of 40 ml/min per 1.73 m2 would result in 312 (2.8%) patients discontinuing metformin and would permit metformin to be used with creatinine concentrations as high as 163 µmol/l.
Conclusions The introduction of eGFR reporting could have a major effect on prescription of metformin. A threshold eGFR of 36–40 ml/min per 1.73 m2 is approximately consistent with our current practice. If our current practice is considered safe, this would be a useful recommendation.