Incidence of Gestational Diabetes Diagnosed on the 100-G Glucose Tolerance Test in Pregnancies with a 50-G Glucose Challenge Test Result of 200 mg/dL or Greater

Objective This study aimed to assess the positive predictive value (PPV) of a 1-hour, 50-g glucose challenge test (GCT) result >= 200 mg/dL for the diagnosis of gestational diabetes mellitus (GDM) on a 3-hour, 100-g glucose tolerance test (GTT). Study Design Pregnancies between 2008 and 2016 with a GCT result >= 200 mg/dL were identified retrospectively. GCT and GTT dates and results, demographics, and working due date (EDD) were extracted. Gestational age at testing was calculated from test date and EDD. As some clinicians presumptively diagnose GDM in such cases, if a GTT result was not available, clinic notes were reviewed to determine whether a GTT was ordered. Positive predictive values (PPV) were calculated at GCT cut-offs at and beyond 200 mg/dL. Subgroups were compared including early GCT (<16 weeks) versus routine GCT (24-28 weeks), GTT result normal versus GTT diagnostic of GDM, and GTT ordered versus GTT not ordered. Rates of use of medication for glycemic control were assessed among these groups. Results Of 236 pregnant women with a GCT result >= 200 mg/dL, 115 (48%) GTT was ordered for 115 (49%), whereas 123 (52%) were managed as presumed GDM. Of 100 (87%) who completed the test, 81 (81%) were diagnosed with GDM with a median intertest interval of 14 days. No statistically significant differences were found between groups stratified by GTT result. Use of rates of metformin, glyburide, and insulin were similar between those diagnosed with GDM by GTT and those diagnosed with GDM by GCT alone. Conclusion A GCT result of >= 200 mg/dL has a PPV of 81% for diagnosis of GDM by GTT in a contemporary U.S. population, with a median intertest interval of 14 days between GCT and GTT. However, those diagnosed by GCT alone were as likely as those diagnosed by GTT to require medication for glycemic control, including insulin, suggesting that requiring a GTT may result in underdiagnosis and delayed treatment of GDM.