When should a patient with a positive initial Gestational Diabetes Mellitus (GDM) screen undergo a subsequent 2-3 hour Oral Glucose Tolerance Test (OGTT)?

From the Guidelines

If the initial gestational diabetes screen is positive, the patient should be ordered to take the next 2-3 hour test as soon as possible, ideally within one week of the abnormal screening result, to promptly diagnose and manage gestational diabetes, reducing the risk of complications for both mother and baby. The 2-3 hour test, also known as the oral glucose tolerance test (OGTT), should be performed in the morning after an overnight fast of at least 8 hours but not more than 14 hours, as recommended by the American Diabetes Association (ADA) in their 2025 standards of care in diabetes 1. Prior to the test, patients should maintain their usual diet with adequate carbohydrate intake (at least 150g per day) for three days before testing. During the test, the patient will have a fasting blood glucose measurement, then drink a solution containing 100g of glucose, followed by blood glucose measurements at 1,2, and 3 hours after consumption.

The diagnosis of gestational diabetes mellitus (GDM) is made if at least two of the following four plasma glucose levels (measured fasting and 1,2, and 3 hours during OGTT) are met or exceeded, according to the Carpenter-Coustan criteria:

• Fasting: 95 mg/dL (5.3 mmol/L)
• 1 hour: 180 mg/dL (10.0 mmol/L)
• 2 hours: 155 mg/dL (8.6 mmol/L)
• 3 hours: 140 mg/dL (7.8 mmol/L) 1. This prompt follow-up testing is crucial because untreated gestational diabetes can lead to complications for both mother and baby, including macrosomia, birth trauma, neonatal hypoglycemia, and increased risk of cesarean delivery. The 2-3 hour OGTT provides a definitive diagnosis of gestational diabetes, allowing for timely intervention with dietary modifications, blood glucose monitoring, and possibly medication if needed.

Key points to consider when ordering the next 2-3 hour test include:

• The test should be performed as soon as possible after the initial positive screen
• The patient should fast for at least 8 hours before the test
• The patient should consume a solution containing 100g of glucose during the test
• Blood glucose measurements should be taken at 1,2, and 3 hours after glucose consumption
• The diagnosis of GDM should be made based on the Carpenter-Coustan criteria, as recommended by the ADA 1.

From the Research

Next Steps After a Positive Initial Gestational Diabetes Screen

• If the initial gestational diabetes screen is positive, the patient should be ordered to take the next 2-3 hour test, also known as the oral glucose tolerance test (OGTT) 2, 3, 4, 5, 6.
• The OGTT is typically performed in the morning after an overnight fast of at least 8 hours 2, 3, 4, 5, 6.
• During the OGTT, the patient's blood glucose levels are measured at fasting, and then 1,2, and sometimes 3 hours after consuming a glucose-rich drink 2, 3, 4, 5, 6.
• The results of the OGTT are used to diagnose gestational diabetes mellitus (GDM) and to determine the best course of treatment 2, 3, 4, 5, 6.

Factors Influencing the Need for Further Testing

• Fasting glucose level on the OGTT is a strong predictor of the need for pharmacotherapy in women with GDM 3, 5.
• Women with higher fasting glucose levels on the OGTT are more likely to require insulin therapy 3, 5.
• The number and type of abnormal OGTT values can also predict the risk of future maternal type 2 diabetes mellitus 4.
• The diagnostic criteria used for GDM, such as the Diabetes Canada or International Association of the Diabetes and Pregnancy Study Groups criteria, can also impact the results of the OGTT 4.