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 are limited data, however, on whether an 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 two urban centers from January 2017 to December 2020 included those who completed both a postpartum FBG prior to discharge and a 2-hour 75-g 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 (e.g., sensitivity, specificity) of postpartum FBG cut-offs to predict an abnormal 2-hour OGTT result.

Results: A total of 235 patients met inclusion criteria, of which 63% were diet-controlled and 37% required medical management. FBG ranged from 64 to 134 mg/dL, with 6/235 (2.6%) with values ≥126 mg/dL. A total of 39/235 (16.6%) of patients had an abnormal 2-hour OGTT. Overall, area under the curve for FBG predicting abnormal 2-hour OGTT was 0.65. Traditionally considered high cut-offs (≥126 mg/dL) for predicting persistent impaired glucose intolerance demonstrated poor positive predictive value (PPV; <20%). In contrast, low cut-offs demonstrated excellent NPV (>90%). A postpartum FBG of 88 mg/dL was determined to be the optimal cut-off 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 an FBG < 88 mg/dL was highly correlative with a normal 2-hour OGTT.

Key points: · Screening for postpartum glucose intolerance is essential for gestational diabetes.. · A promising initiative includes offering screening tests while in the hospital, including an FBG.. · Application of a postpartum FBG < 88 mg/dL is highly correlative with a normal 2-hour OGTT..