Diane Farrar, Lelia Duley, Therese Dowswell, Debbie A Lawlor

Gestational diabetes mellitus (GDM) is carbohydrate intolerance resulting in hyperglycaemia with onset or first recognition during pregnancy. If untreated, perinatal morbidity and mortality may be increased. Accurate diagnosis allows appropriate treatment. Use of different tests and different criteria will influence which women are diagnosed with GDM. This is an update of a review published in 2011 and 2015. To evaluate and compare different testing strategies for diagnosis of gestational diabetes mellitus to improve maternal and infant health while assessing their impact on healthcare service costs. We searched Cochrane Pregnancy and Childbirth's Trials Register, ClinicalTrials.gov, WHO International Clinical Trials Registry Platform (ICTRP) (9 January 2017) and reference lists of retrieved studies. We included randomised trials if they evaluated tests carried out to diagnose GDM. We excluded studies that used a quasi-random model, cluster-randomised or cross-over trials. Two review authors independently assessed trials for inclusion and risk of bias, extracted data and checked them for accuracy. The quality of the evidence was assessed using the GRADE approach. We included a total of seven small trials, with 1420 women. One trial including 726 women was identified by this update and examined the two step versus one step approach. These trials were assessed as having varying risk of bias, with few outcomes reported. We prespecified six outcomes to be assessed for quality using the GRADE approach for one comparison: 75 g oral glucose tolerance test (OGTT) versus 100 g OGTT; data for only one outcome (diagnosis of gestational diabetes) were available for assessment. One trial compared three different methods of delivering glucose: a candy bar (39 women), a 50 g glucose polymer drink (40 women) and a 50 g glucose monomer drink (43 women). We have included the results reported by this trial as separate comparisons. No trial reported on measures of costs of health services.We examined six main comparisons. 75 g OGTT versus 100 g OGTT (1 trial, 248 women): women who received 75 g OGTT had a higher relative risk of being diagnosed with GDM (risk ratio (RR) 2.55, 95% confidence interval (CI) 0.96 to 6.75; very-low quality evidence). No data were reported for the following additional outcomes prespecified for GRADE assessment: caesarean section, macrosomia > 4.5 kg or however defined in the trial, long-term type 2 diabetes maternal, long-term type 2 diabetes infant and economic costs. Candy bar versus 50 g glucose monomer drink (1 trial, 60 women): more women receiving the candy bar, rather than glucose monomer, preferred the taste of the candy bar (RR 0.60, 95% CI 0.42 to 0.86) and 1-hour glucose was less with the candy bar. There were no differences in the other outcomes reported (maternal side effects). No infant outcomes were reported or any review primary outcomes. 50 g glucose polymer drink versus 50 g glucose monomer drink (3 trials, 239 women): mean difference (MD) in gestation at birth was -0.80 weeks (1 trial, 100 women; 95% CI -1.69 to 0.09). Total side effects were less common with the glucose polymer drink (1 trial, 63 women; RR 0.21, 95% CI 0.07 to 0.59), and no clear difference in taste acceptability was reported (1 trial, 63 women; RR 0.99, 95% CI 0.76 to 1.29). Fewer women reported nausea following the 50 g glucose polymer drink compared with the 50 g glucose monomer drink (1 trial, 66 women; RR 0.29, 95% CI 0.11 to 0.78). No other measures of maternal morbidity or outcomes for the infant were reported. 50 g glucose food versus 50 g glucose drink (1 trial, 30 women): women receiving glucose in their food, rather than as a drink, reported fewer side effects (RR 0.08, 95% CI 0.01 to 0.56). No clear difference was noted in the number of women requiring further testing (RR 0.14, 95% CI 0.01 to 2.55). No other measures of maternal morbidity or outcome were reported for the infant or review primary outcomes. 75 g OGTT World Health Organization (WHO) criteria versus 75 g OGTT American Diabetes Association (ADA) criteria (1 trial, 116 women): no clear differences in included outcomes were observed between women who received the 75 g OGTT and were diagnosed using criteria based on WHO (1999) recommendations and women who received the 75 g OGTT and were diagnosed using criteria recommended by the ADA (1979). Outcomes measured included diagnosis of gestational diabetes (RR 1.47, 95% CI 0.66 to 3.25), caesarean section (RR 1.07, 95% CI 0.85 to 1.35), macrosomia defined as > 90th percentile by ultrasound or birthweight equal to or exceeding 4000 g (RR 0.73, 95% CI 0.19 to 2.79), stillbirth (RR 0.49, 95% CI 0.02 to 11.68) and instrumental birth (RR 0.21, 95% CI 0.01 to 3.94). No other secondary outcomes were reported. Two-step approach (50 g oral glucose challenge test followed by selective 100 g OGTT Carpenter and Coustan criteria) versus one-step approach (universal 75 g OGTT ADA criteria) (1 trial, 726 women): women allocated the two-step approach had a lower risk of being diagnosed with GDM at 11 to 14 weeks' gestation compared to women allocated the one-step approach (RR 0.51, 95% CI 0.28 to 0.95). No other primary or secondary outcomes were reported. There is insufficient evidence to suggest which strategy is best for diagnosing GDM. Large randomised trials are required to establish the best strategy for correctly identifying women with GDM.