Objective: To estimate the optimal screen-positive 1-hour 50 gm glucose challenge test (GCT) threshold for gestational diabetes (GDM) and predictive characteristics of increasing screen-positive GCT threshold values (135-199 mg/dL) for GDM.
Study Design: Secondary analysis of a multicenter mild GDM study. At 24-30 weeks' gestation, women with elevated GCT (135-199 mg/dL) completed a diagnostic 3-hour oral glucose tolerance test (OGTT). A novel change-point analysis method was used to compare the GDM rates for adjacent GCT values, delineating categories of changing risk such that values within categories have equal risk for GDM. Positive (PPV) and negative (NPV) predictive values for GDM were computed for increasing GCT cut-offs.
Results: In 7280 women with both GCT (135-199 mg/dL) and OGTT results, 4 GDM risk-equivalent GCT categories were identified with escalations at 144, 158, and 174 mg/dL (all p-values <0.05). The PPV for GDM increased from 33% to 64% as GCT increased from 135 to 199 mg/dL while the NPV decreased from 80% to 67%. PPVs were only 20% and 61% for risk-equivalent categories of 135-143 and 174-199 mg/dL respectively.
Conclusion: Elevated GCT cut-off values between 135-143 mg/dL may carry equivalent GDM risk. No threshold GCT value <199 mg/dL alone sufficiently predicts GDM.