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Laboratory diagnosis of gestational diabetes mellitus

Worldwide, gestational diabetes mellitus (GDM) is a common and serious pregnancy complication with potentially lifelong consequences for the woman and her offspring. There is remarkably little consensus, however, about the diagnosis of GDM despite the publication of the Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) study. The HAPO study aimed to clarify the risk of adverse outcomes associated with maternal glucose intolerance less severe than thresholds for GDM, in the absence of treatment. After excluding 1.8% with overt DM at the time of a 75-g oral glucose tolerance test (OGTT) in 15 centres, the odds ratios were calculated for adverse pregnancy outcomes. There was a continuous association between maternal glucose levels and increased birthweight and cord blood serum C-peptide levels. Maternal glycaemia was more weakly associated with primary caesarean section and clinical neonatal hypoglycaemia. One feature of the HAPO study was the rigorous standardisation of glucose measurements and the integration of local and central laboratory functions. This was a challenge for such a large study because of the wide geographical dispersion of the centres. After publication, the International Association of Diabetes and Pregnancy Study Groups (IADPSG) convened a meeting of over 220 delegates, including authors of the HAPO study. Based on thresholds from the HAPO study that identified pregnancies in which the risks of various adverse outcomes were increased with an odds ratio of 1.75 compared with mean glucose concentrations, a sub-group recommended new criteria for the diagnosis of GDM. They recommended a one-step 2-hour 75-g OGTT, which is accepted for screening outside pregnancy, lower thresholds for abnormal plasma glucose measurements and that the diagnosis could be made on the basis of one abnormal result, and not two as previously. Applying the new criteria post hoc to the women who participated in the HAPO study meant that 17.1% would have been diagnosed with GDM, in addition to the 1.8% excluded. These criteria were adopted subsequently by the World Health Organization. However, agreement among international healthcare organisations has not been universal. The American Congress of Obstetricians and Gynecologists recommends the two-step approach and the 100-g OGTT with higher thresholds for abnormality. In 2015, the National Institute for Health and Care Excellence (NICE) published guidelines for the diagnosis of GDM based on the 2-hour 75-g OGTT. After reviewing data from the four UK HAPO centres and data from a Norwich study, however, NICE recommended a higher fasting threshold for abnormality based on cost-effectiveness. Despite reviewing the same scientific evidence, consensus among experts about the diagnosis of GDM remains elusive. However, the debate has focused on the clinical diagnosis and effective management of GDM, and the associated economic costs. To date, scant attention has been given to the important laboratory components of the diagnosis of GDM. Laboratory glucose measurements are well-established and are based on enzymatic reactions involving one of four enzymes: glucose 1-dehydrogenase, glucose oxidase, glucokinase, or hexokinase. The most common methods of glucose analysis use one of the latter three enzymes. Glucose oxidase is the most specific enzyme and reacts only with D-glucose. The glucokinase or hexokinase methods are considered more accurate than the glucose oxidase method. Glucose oxidase or glucose 1-dehydrogenase are usually used for point-of-care tests. In recent years, however, the influences of preanalytical effects on laboratory results have received considerable attention. The effects can be categorised broadly into physiological, sample collection and other influences. These effects are particularly important in the measurement of