Interpretation of a Three-Hour Glucose Tolerance Test
The three-hour glucose tolerance test (GTT) should be interpreted by comparing blood glucose values at fasting, 1-hour, 2-hour, and 3-hour time points against established diagnostic thresholds, with gestational diabetes diagnosed when two or more values are abnormal. 1
Standard Diagnostic Criteria for 3-Hour GTT (100g glucose load)
The 3-hour GTT is primarily used for diagnosing gestational diabetes mellitus (GDM) as part of a two-step approach:
- First step: 50g glucose load test (GLT) without fasting
- Second step: If 1-hour value is ≥140 mg/dL, proceed to 3-hour 100g OGTT
Diagnostic Thresholds for GDM (3-hour 100g OGTT)
GDM is diagnosed when two or more of the following values are met or exceeded:
| 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 of Single Abnormal Values
Even a single abnormal value on the 3-hour GTT has clinical significance:
- Women with one abnormal glucose value have significantly worse pregnancy outcomes compared to those with normal values, including:
- Increased risk of macrosomia (OR 1.59)
- Higher rates of large-for-gestational-age infants (OR 1.38)
- Greater risk of neonatal hypoglycemia (OR 1.88)
- Increased cesarean delivery rates (OR 1.69)
- Higher rates of pregnancy-induced hypertension (OR 1.55)
- These risks are comparable to those seen in women with diagnosed GDM 2
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. 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)
Test Administration Guidelines
For accurate results:
- The test should be performed in the morning after an overnight fast of at least 8 hours
- For GDM screening, testing should be performed between 24-28 weeks of gestation
- The patient should remain seated and not smoke during the test
Interpretation Considerations
Hypoglycemia at 3 hours: Values below the 5th percentile at the 3-hour mark have been associated with small-for-dates infants and small-for-dates placentas 3
Glucose challenge test (GCT) follow-up: Patients with an elevated 1-hour glucose screen but negative 3-hour GTT still have a 16% chance of developing GDM later in pregnancy and should be considered for repeat testing 4
Glucose tolerance test periodicity: The time it takes for glucose levels to return to baseline is significantly longer in patients with GDM compared to non-diabetic subjects, regardless of glucose load 5
Clinical Management Implications
- Women diagnosed with GDM should receive dietary counseling, home glucose monitoring, and possibly insulin therapy
- Treatment of women with abnormal glucose challenge tests but normal OGTTs has been shown to decrease the prevalence of macrosomia 6
- Women with GDM should be screened for diabetes 6-12 weeks postpartum using non-pregnant OGTT criteria 1
Important Caveats
- The 3-hour reading, while sometimes considered optional, provides valuable information about hypoglycemic responses that may have clinical significance 3
- Marked discrepancies between measured glucose levels and clinical presentation should prompt consideration of other factors affecting glucose metabolism
- In conditions with increased red blood cell turnover (sickle cell disease, pregnancy, hemodialysis, recent blood loss/transfusion), only plasma glucose criteria should be used 7
Remember that the relationship between maternal glucose levels and adverse outcomes appears to be continuous, with no clear inflection points, suggesting that even borderline values may have clinical significance 7.