Normal Range for Three-Hour Glucose Tolerance Test (GTT)
For the three-hour 100g oral glucose tolerance test (OGTT) used in gestational diabetes screening, normal values should be below 95 mg/dL (5.3 mmol/L) at fasting, below 180 mg/dL (10.0 mmol/L) at 1 hour, below 155 mg/dL (8.6 mmol/L) at 2 hours, and below 140 mg/dL (7.8 mmol/L) at 3 hours. 1
Gestational Diabetes Screening Methods
The three-hour glucose tolerance test is primarily used in pregnancy as part of the "two-step" approach for diagnosing gestational diabetes mellitus (GDM). This approach involves:
- Initial screening: 50g glucose load test (GLT) without fasting, with plasma glucose measurement at 1 hour
- Follow-up testing: If the 1-hour value is ≥140 mg/dL (7.8 mmol/L), proceed to the 3-hour 100g OGTT
Three-Hour OGTT Normal Values (Carpenter-Coustan Criteria)
When using the three-hour 100g OGTT, the diagnosis of GDM is made when at least two of the following values are met or exceeded 1:
| Time Point | Threshold Value |
|---|---|
| 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) |
Clinical Significance
Having even a single abnormal value on the three-hour OGTT is associated with adverse pregnancy outcomes. Research shows that women with one abnormal glucose value have significantly worse pregnancy outcomes compared to those with all normal values, with increased risks of 2:
- Macrosomia (OR 1.59)
- Large for gestational age infants (OR 1.38)
- Neonatal hypoglycemia (OR 1.88)
- Cesarean delivery (OR 1.69)
- Pregnancy-induced hypertension (OR 1.55)
Alternative Diagnostic Approach
The American Diabetes Association also recognizes a "one-step" approach using a 75g OGTT with measurements at fasting, 1 hour, and 2 hours 1. In this approach, GDM is diagnosed when any of these values are met or exceeded:
- Fasting: ≥92 mg/dL (5.1 mmol/L)
- 1 hour: ≥180 mg/dL (10.0 mmol/L)
- 2 hour: ≥153 mg/dL (8.5 mmol/L)
Important Considerations
- The three-hour OGTT should be performed in the morning after an overnight fast of at least 8 hours 1
- The American College of Obstetricians and Gynecologists notes that some clinicians may diagnose GDM with just one elevated value rather than requiring two 1
- The test should be performed between 24-28 weeks of gestation for optimal screening 1
Pitfalls to Avoid
- Improper preparation: Ensure patients fast properly before the test (at least 8 hours)
- Timing errors: Accurate timing of blood draws is critical as glucose values change rapidly
- Misinterpretation: Remember that the three-hour 100g OGTT requires at least two abnormal values for GDM diagnosis in most guidelines, though some clinicians use a single abnormal value
- Failure to follow up: Women with GDM should be screened for diabetes 6-12 weeks postpartum using non-pregnant OGTT criteria 1
For non-pregnant adults, the three-hour OGTT is not typically used; instead, a two-hour 75g OGTT is the standard, with diabetes diagnosed at a 2-hour value ≥200 mg/dL (11.1 mmol/L) 1.