Normal Values for 75-Gram Oral Glucose Tolerance Test
For non-pregnant adults, a normal 2-hour glucose value after a 75-gram oral glucose load is less than 140 mg/dL (7.8 mmol/L). 1
Diagnostic Categories for Non-Pregnant Adults
The 75-gram OGTT results are interpreted as follows:
Normal Glucose Tolerance
Prediabetes (Impaired Glucose Tolerance)
Diabetes Mellitus
Test Procedure
The test requires:
- 8-hour overnight fast before the test 1
- 75 grams of anhydrous glucose dissolved in water 1, 2, 3
- Blood draw exactly 2 hours after beginning to drink the glucose solution 1, 2
- Consumption of at least 150 grams of carbohydrates daily for the 3 days prior to testing to avoid falsely elevated results 3
Important Clinical Considerations
The OGTT is considered the reference standard for diagnosing diabetes and identifies more cases of diabetes and prediabetes compared to fasting glucose or A1C testing alone. 1 However, it has limitations including poor reproducibility and requires patient compliance with fasting requirements.
For diagnosis of diabetes, two abnormal test results are required (either from the same sample using different tests, or the same test repeated on a different day), unless the patient presents with unequivocal hyperglycemia or hyperglycemic crisis. 2, 3
Avoid common pitfalls: Recent physical activity, acute illness, stress, or inadequate carbohydrate intake in the days before testing can falsely elevate glucose levels. 3 Samples must be processed promptly as glycolysis will falsely lower glucose concentrations if samples sit at room temperature. 3
Special Population: Gestational Diabetes
For pregnant women at 24-28 weeks gestation using the one-step approach with a 75-gram OGTT, diagnostic thresholds differ:
- Fasting: ≥92 mg/dL
- 1-hour: ≥180 mg/dL
- 2-hour: ≥153 mg/dL (only one abnormal value needed for diagnosis) 4