What is the Glucose Challenge Test in Pregnancy?
The glucose challenge test (GCT) is a screening test used to identify pregnant women at risk for gestational diabetes mellitus (GDM), involving administration of a 50-gram oral glucose load followed by measurement of plasma glucose at 1 hour, typically performed between 24-28 weeks of gestation. 1
Test Methodology
The GCT is performed as follows:
- The patient receives a 50-gram oral glucose load in a non-fasting state 1
- Plasma glucose is measured exactly 1 hour after glucose ingestion 1
- No fasting is required, making it convenient and easy to implement 2, 3
Interpretation and Diagnostic Thresholds
The GCT serves as the initial screening step in the two-step approach to GDM diagnosis:
- If the 1-hour plasma glucose is ≥140 mg/dL (7.77 mmol/L), the patient proceeds to a diagnostic 100-gram 3-hour oral glucose tolerance test (OGTT) 1
- Some centers use a lower threshold of ≥130 mg/dL (7.21 mmol/L) to increase sensitivity, though this reduces specificity 1
- At the 140 mg/dL threshold, the GCT has 85% sensitivity and 86% specificity for detecting GDM 1
- At the 130 mg/dL threshold, sensitivity increases to 99% but specificity drops to 77% 1
Timing of Screening
Universal screening with the GCT is recommended between 24-28 weeks of gestation for all pregnant women not previously diagnosed with diabetes 1, 2. This timing is optimal because glucose tolerance is most impaired around the 28th week of pregnancy 3.
Women with high-risk factors (BMI ≥30 kg/m², previous GDM, family history of diabetes, or high-risk ethnicity) should be tested at the first prenatal visit using standard diagnostic criteria for overt diabetes 1, 2.
Clinical Significance
The GCT identifies women who warrant further diagnostic testing:
- Women with abnormal GCT results (even without confirmed GDM) have a 2.56-fold increased risk of developing diabetes later in life compared to those with normal results 4
- Women with GCT values of 130-139 mg/dL show increased odds of perinatal complications including shoulder dystocia (OR 2.02), birth trauma (OR 1.47), and low Apgar scores (OR 1.51) 5
- Women with GCT ≥140 mg/dL have higher odds of macrosomia (OR 1.32) and shoulder dystocia (OR 1.68) 5
Two-Step vs. One-Step Approach
The GCT is part of the two-step screening strategy:
- The two-step approach (50-gram GCT followed by 100-gram OGTT if abnormal) is currently supported by the American College of Obstetricians and Gynecologists 2
- The alternative one-step approach uses a 75-gram OGTT directly at 24-28 weeks without initial screening 2, 6
- The two-step approach identifies approximately 5-6% of pregnancies as having GDM, while the one-step approach identifies 15-20% 7
Important Clinical Considerations
Universal screening is now standard practice in the United States, with 96% of obstetricians routinely screening all pregnant women for GDM 1. This approach is necessary because risk factor-based screening alone misses a significant proportion of GDM cases 8.
The GCT is simple, cost-effective, user-friendly, and convenient for screening purposes 9. The diagnostic yield when using a 140 mg/dL threshold is approximately 6% 9.