What is the procedure for an oral glucose tolerance test (OGTT)?

Oral Glucose Tolerance Test Procedure

The OGTT should be performed in the morning after an overnight fast of 8-14 hours using a 75g anhydrous glucose load dissolved in water, with venous plasma glucose measurements taken at fasting, 1-hour, and 2-hour time points (or at fasting, 1-hour, 2-hour, and 3-hour for the 100g test in pregnancy). 1

Pre-Test Patient Preparation

• Dietary requirements: The patient must consume at least 150g of carbohydrate per day for a minimum of 3 days before the test 1, 2
• Fasting duration: No caloric intake for at least 8 hours (some guidelines specify 8-14 hours) before the test 1, 2
• Physical activity: Maintain unlimited, unrestricted physical activity in the days leading up to the test 1, 2
• Timing: The test must be performed in the morning 1, 2, 3

Test Administration

• Glucose load: Administer 75g of anhydrous glucose dissolved in water for standard diabetes diagnosis 1
• Patient positioning: The patient must remain seated throughout the entire test 1, 2
• Smoking restriction: The patient should not smoke during the test 1, 2
• Blood sampling: Draw venous plasma glucose at fasting (0 hours), 1 hour, and 2 hours after glucose administration 1, 2, 3

Diagnostic Thresholds for Diabetes (Non-Pregnant Adults)

• Fasting plasma glucose: ≥126 mg/dL (7.0 mmol/L) 1
• 2-hour plasma glucose: ≥200 mg/dL (11.1 mmol/L) 1
• Confirmation: In the absence of unequivocal hyperglycemia, abnormal results should be confirmed by repeat testing on a different day 1

Special Considerations for Gestational Diabetes

One-Step Approach (IADPSG Criteria)

• Glucose load: 75g with measurements at fasting, 1-hour, and 2-hour 2, 3
• Diagnostic thresholds: Fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, or 2-hour ≥153 mg/dL (any single value meets diagnosis) 2, 3
• Timing: Perform at 24-28 weeks of gestation 2, 3

Two-Step Approach (Carpenter-Coustan Criteria)

• Initial screening: 50g glucose challenge test (non-fasting) at 24-28 weeks 4, 2
• Diagnostic test: If screening positive, perform 100g OGTT with measurements at fasting, 1-hour, 2-hour, and 3-hour 4, 2
• Diagnostic thresholds: At least two of the following must be met or exceeded: fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL, 3-hour ≥140 mg/dL 1, 4, 2, 3

Important Clinical Pitfalls

• Laboratory requirements: The test must be performed in a certified laboratory using NGSP-certified methods standardized to the DCCT assay; point-of-care assays should not be used for diagnosis 1
• High-risk pregnant women: Those with marked obesity, personal history of GDM, glycosuria, or strong family history should be tested at the first prenatal visit (12-14 weeks), and if negative, retested at 24-28 weeks 1, 2
• Body weight considerations: Approximately 10% of patients may have discordant results between fasting and 2-hour values, with differences noted in body weight and height potentially affecting test accuracy 5
• Patient tolerance: The OGTT may cause rebound hypoglycemia and gastrointestinal discomfort; patients who cannot tolerate oral glucose loads should not undergo this test 3, 6