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OBJECTIVE To assess the positive predictive value of a one-hour, 50-gram glucose challenge test (GCT) result ≥200mg/dL for the diagnosis of gestational diabetes (GDM) on a 3-hour, 100-gram glucose tolerance test (GTT). STUDY DESIGN Pregnancies 2008-2016 with a GCT result ≥200mg/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 value was calculated at GCT cutoffs at and beyond 200mg/dL. Sub-groups of interest were compared including early GCT (<16 weeks) vs routine GCT (24-28 weeks), GTT passed vs GTT diagnostic of GDM, and GTT ordered vs GTT not ordered. Rates of use of medication for glycemic control were assessed among these groups. RESULT GTT was ordered for 115 of 236 pregnancies with a GCT result ≥200mg/dL (49%). Of 100 (87%) who completed the test, 81 (81%) were diagnosed with GDM with a median inter-test 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 ≥200mg/dL has a positive predictive value of 81% for diagnosis of GDM by GTT in a contemporary US population, with a median inter-test 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 under-diagnosis and delayed treatment of GDM.