What are the optimal results for a 3-hour glucose tolerance test (GTT)?

Optimal Results for a 3-Hour Glucose Tolerance Test

For a standard 3-hour glucose tolerance test using a 100g glucose load, normal values should be below: fasting 95 mg/dL, 1-hour 180 mg/dL, 2-hour 155 mg/dL, and 3-hour 140 mg/dL. 1

Standard 3-Hour Glucose Tolerance Test (100g)

The 3-hour glucose tolerance test is most commonly used as part of the "two-step" approach for diagnosing gestational diabetes mellitus (GDM). Normal values for this test are:

• Fasting: < 95 mg/dL (5.3 mmol/L)
• 1 hour: < 180 mg/dL (10.0 mmol/L)
• 2 hour: < 155 mg/dL (8.6 mmol/L)
• 3 hour: < 140 mg/dL (7.8 mmol/L)

These thresholds are based on the Carpenter-Coustan criteria, which are widely accepted in clinical practice 1, 2.

Interpretation of Results

Normal Results

• All values below the thresholds listed above indicate normal glucose tolerance.

Abnormal Results

• For gestational diabetes diagnosis: Two or more values meeting or exceeding the thresholds indicate GDM 1.
• Some clinicians and the American College of Obstetricians and Gynecologists note that even one elevated value may be used for diagnosis, as research shows increased risk with single abnormal values 2, 3.

Clinical Significance

Research has demonstrated that even a single abnormal value on a 3-hour GTT is associated with adverse outcomes:

• Increased risk of macrosomia (odds ratio 1.59)
• Higher rates of large-for-gestational-age infants (odds ratio 1.38)
• Greater risk of neonatal hypoglycemia (odds ratio 1.88)
• Higher cesarean delivery rates (odds ratio 1.69)
• Increased pregnancy-induced hypertension (odds ratio 1.55) 3

Test Administration Guidelines

For accurate results, the 3-hour GTT should be:

• Performed in the morning after an overnight fast of at least 8 hours
• Patient should remain seated during the test
• No smoking during the test
• For gestational diabetes screening, testing should be performed between 24-28 weeks of gestation 2

Alternative Glucose Tolerance Tests

75g Oral Glucose Tolerance Test (2-hour)

Used in the "one-step" approach for GDM diagnosis with these thresholds:

• 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) 1

Non-Pregnancy Diabetes Diagnosis

For non-pregnant adults, diabetes is diagnosed with a 75g OGTT if:

• 2-hour plasma glucose ≥ 200 mg/dL (11.1 mmol/L)
• Research suggests a 1-hour threshold of ≥ 11.6 mmol/L (208.8 mg/dL) has good sensitivity (92%) and specificity (91%) for detecting type 2 diabetes 4

Important Considerations

• GTT periodicity (time to return to baseline) is significantly longer in patients with diabetes compared to those without diabetes, regardless of glucose load 5
• In conditions with increased red blood cell turnover (sickle cell disease, pregnancy, hemodialysis, recent blood loss/transfusion), only plasma glucose criteria should be used rather than HbA1c 2
• The relationship between maternal glucose levels and adverse outcomes appears to be continuous, with no clear inflection points 2

Pitfalls to Avoid

• Failing to adhere to proper fasting requirements before the test
• Not maintaining proper timing for blood draws (exactly at fasting, 1-hour, 2-hour, and 3-hour marks)
• Misinterpreting a single abnormal value as normal, when research shows increased risks even with one elevated value 3
• Not considering that glucose challenge test values of 140 mg/dL or higher are associated with increased odds of adverse perinatal outcomes, even when subsequent GTT results are normal 6