What is OGGT?

What is OGTT?

OGTT stands for Oral Glucose Tolerance Test, a diagnostic procedure used to evaluate how the body processes glucose by measuring blood glucose levels at specific time intervals after consuming a standardized glucose load. 1

Test Administration and Preparation

The OGTT requires specific patient preparation to ensure accurate results:

• The patient must fast for 8-14 hours before the test (typically performed in the morning after an overnight fast). 1
• At least 150 grams of carbohydrate must be consumed daily for 3 days prior to testing to ensure adequate glycogen stores. 1, 2
• The patient must remain seated throughout the entire test and should not smoke. 1, 2
• Unlimited physical activity should be maintained in the days leading up to the test. 2

Standard Glucose Loads and Measurement Points

For non-pregnant adults, a 75-gram glucose load is administered, with blood glucose measurements taken at fasting and 2 hours post-ingestion. 1

For pregnant women (gestational diabetes screening), two approaches exist:

One-Step Approach (IADPSG Criteria)

• Uses a 75-gram glucose load with measurements at fasting, 1-hour, and 2-hour time points. 2
• Diagnostic thresholds: fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥153 mg/dL. 2
• Identifies approximately 15-20% of pregnant women with gestational diabetes. 2

Two-Step Approach (ACOG-Supported)

• Initial 50-gram glucose challenge test, followed by 100-gram OGTT if positive. 2, 3
• Diagnostic thresholds (Carpenter/Coustan): fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL, 3-hour ≥140 mg/dL. 2
• Identifies approximately 5-6% of pregnant women with gestational diabetes. 2

Diagnostic Criteria for Diabetes

In non-pregnant adults, a 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during OGTT confirms diabetes (must be confirmed on a subsequent day unless unequivocal hyperglycemia is present). 1

For impaired glucose tolerance (IGT), the 2-hour value falls between 140-199 mg/dL (7.8-11.0 mmol/L). 1

Clinical Utility and Limitations

The OGTT is more sensitive than fasting plasma glucose (FPG) for detecting diabetes and pre-diabetes, but it is less reproducible, more costly, and less convenient. 1

For routine clinical practice, FPG is preferred over OGTT due to ease of use, lower cost, and better patient acceptability. 1

However, OGTT may be useful in patients with impaired fasting glucose (IFG) to better define diabetes risk. 1

Important Caveats

• The test has poor reproducibility, particularly for the 2-hour glucose value (κ statistic of 0.157-0.174, indicating only "slight strength of agreement"). 4
• Caution should be exercised when interpreting a single positive OGTT result, especially in pregnancy. 4
• The OGTT is not recommended for routine clinical screening outside of pregnancy due to its practical limitations. 1

Timing for Gestational Diabetes Screening

All pregnant women should be screened at 24-28 weeks of gestation. 2, 3

High-risk women (marked obesity with BMI ≥30 kg/m², personal history of gestational diabetes, glycosuria, or strong family history of diabetes) should undergo glucose testing as early as the first prenatal visit (12-14 weeks). 2, 3

If negative at the first prenatal visit, high-risk women must be retested at 24-28 weeks. 2, 3