The utility of diagnostic tests in the detection and prediction of glucose intolerance in the early and late postpartum period in women after gestational diabetes: a longitudinal cohort study

Background Due to diverging international recommendations, the unclear role of HbA1c and the lack of longitudinal data, we investigated the accuracy of diagnostic tests in the early and late postpartum in women with gestational diabetes (GDM) especially to predict future glucose-intolerance. Methods This longitudinal cohort included 967 women with GDM from 2011 to 2020. A 75-g oGTT and HbA1c were performed at 4-12 weeks (early) postpartum. FPG and HbA1c were measured at 1 and 3-year (late) postpartum. ADA criteria were used as gold standards. At all time-points (4-12 weeks, 1-year and 3-year postpartum) women with diabetes and prediabetes were grouped together and referred to as glucose-intolerant, because at most 3% of the entire cohort population had diabetes at any time-point. Results The prevalence of glucose-intolerance in the early postpartum was higher using FPG and HbA1c (27.5%) than oGTT criteria (18.2%). Only 48-80% of women diagnosed with glucose-intolerance in the early postpartum actually remained intolerant. This was especially low when FPG or oGTT were combined with HbA1c (1-year: <= 62% and 3-years: <= 50%). Regardless of the test used, 1/3 of women with initially normal glucose-tolerance became glucose-intolerant in the late postpartum. HbA1c was unrelated to iron status/intake, remained stable throughout, but poorly predicted future glucose-intolerance. In the longitudinal analyses, all diagnostic tests in the early postpartum showed acceptable specificities (74-96%) but poor sensitivities (all < 38%) to predict glucose-intolerance after only 10-months. At 1-year postpartum however, the combination of FPG and HbA1c could best predict glucose-intolerance 2-years later. Conclusions Combining FPG with HbA1c at 1-year postpartum represents a reliable choice to predict future glucose-intolerance. Given the poor prediction of tests including oGTT in the early postpartum, focus should rather be on continuous long-term screening.