Accuracy of HbA1c Levels as a Screening Tool for Gestational Diabetes Mellitus in Pregnant Women Prior to 20 Weeks of Gestation
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Abstract
Objectives: To determine the diagnostic accuracy of hemoglobin A1c (HbA1c) measured before 20 weeks’ gestation as a screening tool for gestational diabetes mellitus (GDM) in high-risk pregnancies, and to evaluate whether adding HbA1c to the conventional two-step algorithm (50-g glucose-challenge [50 g GCT] test followed by the 100-gram oral glucose tolerance test [100 g OGTT]) improves early detection.
Materials and Methods: This retrospective study 282 pregnant women who met high-risk criteria for GDM. All participants underwent HbA1c measurement and a 50 g GCT at the first antenatal visit. Elevated HbA1c levels (≥ 5.7%) and/or abnormal GCT results (≥ 140 mg/dL) prompted patients to undergo a diagnostic 100g OGTT. Receiver operating characteristic curve analysis was used to identify the optimal HbA1c cutoff and to calculate sensitivity, specificity, PPV, NPV and accuracy. The screening performance of HbA1c was compared with 50 g GCT.
Results: Among 282 high-risk pregnancies, the overall prevalence of GDM was 29.8% (84/282), and 20.2% (57/282) were diagnosed before 20 weeks of gestation. Women with early-pregnancy HbA1c levels of 5.7–6.4% (38/282, 13.5%) had a significantly higher prevalence of GDM compared with those with HbA1c < 5.7% (71.1% vs 23.4%, respectively) and were more likely to require insulin therapy (13.2% vs 1.6%, p < 0.001). ROC analysis for HbA1c yielded an area under the curve of 0.716. Using an HbA1c threshold of ≥ 5.7% provided high specificity (92.9%) and overall accuracy (81.9%) but low sensitivity (38.6%), whereas the 50-g GCT demonstrated higher sensitivity (96.5%) with lower specificity (67.6%) and accuracy (73.4%).
Conclusion: HbA1c measured before 20 weeks in the 5.7–6.4% range could not replace glucose-based testing due to its limited sensitivity. However, with high specificity and an overall accuracy of 81.9%, it served as a valuable adjunct to the 50 g GCT within the two-step screening protocol, especially in cases where glucose loading was intolerant.
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