Fasting blood glucose predicts response to extended-release metformin in gestational diabetes mellitus

Background Metformin is increasingly accepted as an alternative to insulin therapy in gestational diabetes mellitus (GDM). The Metformin in Gestational Diabetes (MiG) trial reported similar pregnancy outcomes for metformin versus insulin; however, supplemental insulin was required in 46% of women on metformin. Aims We aimed to identify predictors of response to metformin monotherapy in women with GDM attending a general hospital antenatal clinic. Methods We offered extended-release metformin to women diagnosed with GDM (ADIPS 1998 criteria) at 24weeks of gestation. If glucose targets were not achieved (5.0mmol/L fasting, 6.7mmol/L two-h post-meal), women were changed to insulin. We carried out an audit to determine characteristics of metformin responders versus nonresponders. Results Twenty-five women chose initial metformin therapy; 16 (64%) achieved satisfactory glycaemic control (responders). Nine women (36%) were changed to insulin: seven due to inadequate control (nonresponders) and two had metformin intolerance. Fasting glucose at oral glucose tolerance test (OGTT) was significantly lower in metformin responders versus nonresponders; two-h glucose and BMI did not differ. Ninety-three percent of women with fasting glucose 5.2mmol/L responded to metformin: conversely, at fasting glucose >5.2mmol/L, 33% responded (P=0.005). Neonatal outcomes were similar in metformin responders and nonresponders, women who chose initial insulin therapy (n=25), or were diet-controlled (n=21). Conclusions In women with GDM, fasting glucose on OGTT predicted response to metformin: at fasting glucose 5.2mmol/L, the probability of response was 93%. Antenatal clinics should determine locally relevant predictors of response to metformin in women with GDM.