What blood glucose level is considered abnormal in a nonfasting 1-hour 50-g glucose challenge test?

Abnormal Thresholds for Nonfasting 1-Hour 50-g Glucose Challenge Test in GDM Screening

The American College of Obstetricians and Gynecologists (ACOG) recommends a threshold of ≥140 mg/dL for the nonfasting 1-hour 50-g glucose challenge test (GCT), though thresholds of 130 mg/dL or 135 mg/dL are also acceptable, particularly in high-risk populations. 1

Two-Step Approach for GDM Screening

• The two-step approach begins with a 50-g glucose load test (GLT) performed in a nonfasting state at 24-28 weeks gestation 1
• If the plasma glucose level measured 1 hour after the load is ≥140 mg/dL (7.8 mmol/L), the patient proceeds to a 100-g OGTT 1
• The 2024 American Diabetes Association guidelines specifically note that thresholds of 130,135, or 140 mg/dL are all acceptable for the first step of the two-step approach 1
• The threshold of 140 mg/dL identifies approximately 80% of women with GDM, while using a lower cutoff of 130 mg/dL increases the yield to about 90% 1

Threshold Variations Based on Risk Factors

• ACOG specifically recommends a lower threshold of 135 mg/dL (7.5 mmol/L) in high-risk ethnic minorities with higher prevalence of GDM 1
• Some experts recommend using 130 mg/dL (7.2 mmol/L) as the threshold for certain populations 1
• A study of high-risk pregnancies found that a GCT value of ≥140 mg/dL showed sensitivity of 95.3% and specificity of 48.6% for detecting GDM 2

Clinical Implications of Different Thresholds

• Using the threshold of 140 mg/dL is associated with increased odds of adverse perinatal outcomes compared to values <120 mg/dL 3
• Glucose challenge test values of 135-139 mg/dL were not associated with adverse outcomes compared with values <120 mg/dL in one study 3
• Even subcategories with glucose challenge test values of 140-144 mg/dL and 145-149 mg/dL were associated with an increase in selected adverse outcomes 3

Important Considerations for Test Administration

• The GCT is designed to be performed in a nonfasting state 1
• One study found no difference in test results between fasted and fed states among normal individuals, but women with known gestational diabetes had significantly higher values when fasted (173.9 mg/dL) than when fed (154.8 mg/dL) 4
• Accurate timing of the 1-hour sample is important, as glucose values can change rapidly and unpredictably within the 50-70 minute window after glucose ingestion 5

Diagnostic Process After Positive Screening

• After a positive GCT result, a diagnostic 100-g OGTT should be performed in the fasting state 1
• The diagnosis of GDM is made when at least two of the following four plasma glucose levels are met or exceeded:
  • Fasting: 95 mg/dL (5.3 mmol/L)
  • 1-hour: 180 mg/dL (10.0 mmol/L)
  • 2-hour: 155 mg/dL (8.6 mmol/L)
  • 3-hour: 140 mg/dL (7.8 mmol/L) 1

Common Pitfalls

• Using different thresholds without considering the population's risk profile may lead to under or over-diagnosis 1
• Inaccurate timing of the 1-hour sample can lead to erroneous interpretation of the GCT 5
• Failure to follow up positive screening results with the diagnostic OGTT may lead to missed diagnoses 1
• Not considering the fasting/fed state when interpreting borderline results may affect clinical decision-making, particularly in high-risk populations 4