Should a 2-hour glucose challenge test be done with the patient fasting before consuming the 50g glucose solution?

Glucose Challenge Test Protocol for Diabetes Screening

The 50g glucose challenge test (GCT) should be performed in a nonfasting state when screening for gestational diabetes mellitus. 1

Standard Protocol for Glucose Challenge Testing

• The 50g glucose challenge test is specifically designed to be performed without fasting, particularly when used as the first step in the two-step approach for gestational diabetes screening 1, 2
• For the 50g GCT, the patient consumes a 50g glucose solution, and blood glucose is measured 1 hour later, without requiring any specific fasting period 1
• The American College of Obstetricians and Gynecologists (ACOG) recommends this test be performed in a nonfasting state at 24-28 weeks gestation for pregnant women 1, 2
• Abnormal threshold values for the nonfasting 50g GCT are typically ≥140 mg/dL, though thresholds of 130 mg/dL or 135 mg/dL are also acceptable depending on population risk factors 1

Evidence Supporting Nonfasting Status for 50g GCT

• Research has demonstrated that prior meal ingestion actually affects the glucose response to the 50g GCT, with glucose levels typically being higher when the test is performed in a fasting state compared to a postprandial state 3
• In non-diabetic pregnant women, plasma glucose concentrations 1 hour after ingestion of a 50g oral glucose load are higher if administered in the fasting state compared to the postprandial state 3
• The time of day and fasting duration can influence glucose levels following a 50g GCT, with some studies showing significantly lower glucose concentrations in certain time-of-day and fasting duration combinations 4

Important Distinction from Other Glucose Tests

• Unlike the 50g GCT, the diagnostic 75g or 100g oral glucose tolerance test (OGTT) must be performed in the morning after an overnight fast of at least 8 hours 5, 2
• For standard diabetes diagnosis using the 75g OGTT, the American Diabetes Association recommends that the test be performed in the morning after an overnight fast of at least 8 hours 5, 2
• The 100g OGTT used in the second step of GDM screening should be performed when the patient is fasting 2

Clinical Implications and Recommendations

• Using the 50g GCT in a nonfasting state is more practical and convenient for patients 2
• Research shows that the 1-hour glucose screen (50g GCT) is more predictive of gestational diabetes than postmeal assessment 6
• When a patient screens positive on the 50g GCT (≥140 mg/dL), they should proceed to the diagnostic 100g OGTT, which must be performed in a fasting state 2, 1

Common Pitfalls to Avoid

• Requiring fasting for the 50g GCT is unnecessary and may actually lead to higher glucose values that could increase false positive rates 3
• Failing to standardize the time of day and fasting duration for the 50g GCT may introduce variability in test results 4
• Confusing the protocol for the screening 50g GCT (nonfasting) with the diagnostic 75g or 100g OGTT (fasting required) can lead to improper test administration 2, 5