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.