How long should I fast for a gestational diabetes (GD) screen?

From the Guidelines

For a gestational diabetes screen, you should fast for at least 8 hours before the test. This typically means not eating or drinking anything except water after midnight if your test is scheduled in the morning. For the standard glucose challenge test (GCT), which is the initial screening, fasting is not always required, but for the diagnostic glucose tolerance test (GTT), fasting is necessary. During the fasting period, you should avoid all food, sweetened drinks, coffee, tea, and gum, but you can drink plain water. Fasting ensures accurate blood glucose measurements by eliminating the influence of recent food intake on your results. The test measures how your body processes glucose, and eating beforehand would interfere with this assessment. Be sure to follow any specific instructions from your healthcare provider, as protocols may vary slightly between practices. Remember to bring a snack to eat after the test is completed, as you may feel hungry or lightheaded.

Key Points to Consider

• The American Diabetes Association recommends an overnight fast of at least 8 hours before the test 1.
• The test should be performed in the morning after an overnight fast of between 8 and 14 h and after at least 3 days of unrestricted diet (150 g carbohydrate per day) and unlimited physical activity 1.
• Different diagnostic criteria will identify different degrees of maternal hyperglycemia and maternal/fetal risk, leading experts to debate optimal strategies for the diagnosis of GDM 1.
• The expected benefits of using IADPSG criteria for offspring are inferred from intervention trials focusing on individuals with lower levels of hyperglycemia than those identified using older GDM diagnostic criteria 1.

Important Considerations

• The diagnosis of GDM is made when any of the following plasma glucose values are met or exceeded: Fasting: 92 mg/dL (5.1 mmol/L), 1 h: 180 mg/dL (10.0 mmol/L), 2 h: 153 mg/dL (8.5 mmol/L) 1.
• The OGTT should be performed in the morning after an overnight fast of at least 8 h 1.
• The American College of Obstetricians and Gynecologists notes that one elevated value can be used for diagnosis 1.

From the Research

Fasting Requirements for Gestational Diabetes Screen

• The length of time to fast for a gestational diabetes (GD) screen can vary depending on the specific testing protocol being used 2, 3.
• For the 1-step approach, which involves a 75 g, 2-hour oral glucose tolerance test, women are typically required to fast for the test 2.
• The International Association of Diabetes and Pregnancy Study Groups recommends that women fast for this test, but the exact duration of fasting is not specified 2.
• A recent study found that fasting for 6 or more hours before the 1-hour oral glucose tolerance test (OGTT) resulted in a higher screen-positive rate compared to eating within 2 hours of the test 3.
• However, the American College of Obstetricians and Gynecologists recommends a 2-step protocol, which includes a 50 g, 1-hour glucose challenge test that can be administered without regard to the time of the last meal 2.

Testing Protocols and Fasting Requirements

• The 2-step protocol does not require fasting for the initial glucose challenge test, but a 100 g, 3-hour oral glucose tolerance test may be required if the screening test threshold is exceeded 2.
• The 1-step approach, on the other hand, requires fasting for the 75 g, 2-hour oral glucose tolerance test, but does not require a second visit and lasts 2 hours rather than 3 2.
• Other studies have suggested that glucose screening at 16 weeks of pregnancy can be a useful alternative to third-trimester screening for gestational diabetes, but the fasting requirements for this test are not specified 4.
• The optimal fasting duration for gestational diabetes screening is not well established, and more research is needed to determine the best approach 3.