Interpreting 3-Hour Glucose Tolerance Test Results in Pregnancy
Normal Ranges for the 100-gram 3-Hour OGTT
For a pregnant woman with no prior history of gestational diabetes, the normal ranges (using Carpenter-Coustan criteria) are: Fasting <95 mg/dL, 1-hour <180 mg/dL, 2-hour <155 mg/dL, and 3-hour <140 mg/dL. 1, 2, 3
Diagnostic Criteria
Gestational diabetes is diagnosed when at least TWO of the four values meet or exceed the thresholds listed above. 1, 3, 4
However, important nuances exist:
The American College of Obstetricians and Gynecologists (ACOG) now supports using a SINGLE elevated value for diagnosis in clinical practice, recognizing that even one abnormal value carries significant risk. 4
This represents an evolution from traditional criteria, as research demonstrates that women with just one abnormal value have significantly increased risks for adverse outcomes including macrosomia (odds ratio 1.59), large for gestational age infants (odds ratio 1.38), neonatal hypoglycemia (odds ratio 1.88), cesarean delivery (odds ratio 1.69), and pregnancy-induced hypertension (odds ratio 1.55). 5
Testing Protocol Requirements
The 3-hour test must be performed under specific conditions to ensure accuracy:
- The test should be done in the morning after an overnight fast of 8-14 hours 1
- The patient should have consumed at least 150g of carbohydrate per day for at least 3 days prior to testing 1
- The patient should remain seated and not smoke throughout the test 1
- Blood samples are drawn at fasting, then at 1,2, and 3 hours after consuming the 100-gram glucose load 1, 2, 3
Clinical Context: Two-Step Screening Approach
The 3-hour test is typically performed as the second step after an abnormal 1-hour 50-gram glucose challenge test:
- The initial screening uses a 50-gram glucose load (non-fasting) with measurement at 1 hour 3
- A threshold of ≥140 mg/dL on the 1-hour screen triggers the diagnostic 3-hour test, though thresholds of 130 or 135 mg/dL are acceptable in high-risk populations 3
Common Pitfalls to Avoid
Do not dismiss a single elevated value as clinically insignificant - current evidence supports that even one abnormal value warrants clinical attention and possibly treatment 4, 5
Ensure proper patient preparation - inadequate carbohydrate intake in the days before testing can produce falsely abnormal results 1
Consider that 16% of women with an elevated 1-hour screen but normal 3-hour test in early pregnancy may develop gestational diabetes later, so repeat testing in the third trimester may be warranted in these cases 6