Utilization of a postpartum fasting blood glucose to predict impaired glucose tolerance in patients with gestational diabetes mellitus

Objective: The Endocrine Society recommends a postpartum fasting blood glucose (FBG) be performed for patients with gestational diabetes mellitus (GDM) prior to hospital discharge to screen for ongoing hyperglycemia. There is limited data, however, on whether a FBG can screen for glucose intolerance and if it correlates with the gold standard 4-to-12-week 2-hour oral glucose tolerance test (OGTT). Our objective was to evaluate if FBG correlates with the gold standard 2-hour OGTT. Study Design: This retrospective cohort study of patients with GDM who delivered >20 weeks gestation at 2 urban centers from January 2017 to December 2020 included those who completed both a postpartum FBG prior to discharge and a 2-hour 75-gram OGTT within 1 year of delivery. Abnormal 2-hour OGTT was defined as fasting value ≥100mg/dL and/or 2-hour value ≥140mg/dL. We evaluated test characteristics (e.g. sensitivity, specificity) of postpartum FBG cutoffs at predicting an abnormal 2-hour OGTT result. Results: 235 patients met inclusion criteria, of which 63% were diet-controlled and 37% required medical management. FBG ranged from 64-134mg/dL, with 6/235 (2.6%) with values ≥ 126mg/dL. 39/235 (16.6%) of patients had an abnormal 2-hour OGTT. Overall, AUC for FBG predicting abnormal 2-hour OGTT was 0.65. Traditionally considered high cutoffs (≥126mg/dL) for predicting persistent impaired glucose intolerance demonstrated poor PPV (< 20%). In contrast, low cutoffs demonstrated excellent NPV (>90%). A postpartum FBG of 88mg/dL was determined to be the optimal cutoff for FBG with NPV=92.4% (Youden index=0.34). In this dataset, if FBG ≥88mg/dL was used to determine if 2-hour OGTT was required, almost half of GDM patients could avoid further glucose tolerance testing. Conclusions: While previously thought of as best utilized for its PPV, the FBG may be best used for its NPV. In our study, clinical application of a FBG <88mg/dL was highly correlative with a normal 2-hour OGTT.