(939) Utilization of a postpartum fasting blood glucose to predict impaired glucose tolerance in gestational diabetics

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. Our objective was to evaluate if FBG correlates with the gold standard 6 week 2-hour oral glucose tolerance test (OGTT), and if so, define cut-offs for clinical use.

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 ≥ 100 mg/dL and/or 2-hour value ≥ 140 mg/dL. We evaluated test characteristics (sensitivity, specificity, positive predictive value [PPV], negative predictive value [NPV], area under the curve (AUC), and Youden index) 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 medications.  FBG ranged from 64-134mg/dL, with 6/235 (2.6%) with values ≥ 126mg/dL and 39/235 (16.6%) with an abnormal 2-hour OGTT.  Overall, AUC for FBG predicting abnormal 2-hour OGTT was 0.65. Test characteristics for considered cut-off points are demonstrated in the Table. Traditionally considered high cutoffs (ie 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≥ 88 mg/dL was used to determine if 2-hour OGTT was required, almost half of GDM patients could avoid further glucose tolerance testing.

Conclusion: 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. Further investigation in a larger cohort is warranted.