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While ACOG recommends screening obese women for gestational diabetes (GDM) early, standards for early screening are not established. Many providers use the same GDM screening and diagnostic criteria regardless of gestational age, which may not be appropriate due to increasing insulin resistance throughout pregnancy. We hypothesize that lower screening and diagnostic thresholds are needed at 14-20 wks to identify GDM.
Planned secondary analysis of an RCT. Eligible women (BMI≥30 kg/m2) were enrolled <20 wks and randomized to GDM testing (1-hr, 50-g glucose challenge test (GCT) followed by a 3-hr, 100-g glucose tolerance test (GTT) if GCT≥135 mg/dL) at 14-20 wks versus 24-28 wks. Carpenter-Coustan criteria were used to diagnose GDM. This analysis included only women with 14-20 wk and 24-28 wk GCT available. A receiver operator characteristics (ROC) curve was created to assess the test characteristics of early testing compared to GDM >24 wks (gold standard). The Liu method was used to determine the ideal cutoff for each test. The test characteristics of various cutoffs were determined. The incidence of a composite adverse outcome (>4000g, primary cesarean, pregnancy-induced hypertension, hyperbilirubinemia, and hypoglycemia) above and below cutoffs were compared using a chi- squared test.
Of 912 women completing the RCT, 319 (35%) had a GCT available at both time points and 30 (9.4%) were diagnosed with GDM >24 wks. 68 women had a GTT performed at 14-20 weeks. The gestational age at first screen was 17.3±1.7 wks and 26.2±1.3 wks at second. GCT at 14-20 wks was closely associated with GDM at 24-28 wks (area under the curve [AUC] 0.80) with an optimal cutoff of 130 mg/dL. Compared to the cutoff of 135 mg/dL, lowering the threshold to 130 mg/dL increases the sensitivity from 63% to 70% and increases the number of GTTs performed by 4/100 patients. Early GCT≥ 130 mg/dL was associated with the primary outcome (p=0.04). Liu cutoffs for early GTT diagnosis of GDM were 97 (fasting), 155 (1-hr), 127 (2-hr), and 95 (3-hr). Using these cutoffs, an additional 37 women would be diagnosed early with GDM: 43% were diagnosed with GDM at 24-28 wks and 75% had the adverse composite outcome.
Early screening for GDM may require lower cutoffs than those used at 24-28 wks. Information regarding whether or not outcomes are improved at these levels is needed before implementing new screening cutoffs.