3-Hour Glucose Tolerance Test: Normal Blood Glucose Thresholds
For the 3-hour glucose tolerance test (100-gram glucose load), the normal 3-hour reading should be below 140 mg/dL (7.8 mmol/L), with gestational diabetes diagnosed when this value meets or exceeds 140 mg/dL using Carpenter-Coustan criteria. 1
Diagnostic Criteria for the 3-Hour Test
The 3-hour oral glucose tolerance test uses a 100-gram glucose load and requires four blood glucose measurements 1:
- 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)
Gestational diabetes is diagnosed when at least two of these four values are met or exceeded 1. The American College of Obstetricians and Gynecologists notes that a single elevated value can also be used for diagnosis 1.
Clinical Context: Two-Step Screening Approach
The 3-hour test is part of the two-step screening strategy for gestational diabetes 1, 2:
Step 1: A 50-gram, 1-hour glucose challenge test (non-fasting) is performed at 24-28 weeks of gestation 1. If the 1-hour plasma glucose is ≥130,135, or 140 mg/dL (depending on institutional protocol), proceed to Step 2 1.
Step 2: The 100-gram, 3-hour oral glucose tolerance test is performed in the fasting state 1. The patient must fast for at least 8 hours before the test 1.
Alternative Diagnostic Criteria
An older set of criteria (National Diabetes Data Group) uses slightly different thresholds for the 3-hour value 1:
- 3-hour threshold: 145 mg/dL (8.0 mmol/L) instead of 140 mg/dL
However, the Carpenter-Coustan criteria with the 140 mg/dL threshold at 3 hours are more commonly used in current practice 1.
Important Clinical Considerations
The 3-hour test is only used for diagnosing gestational diabetes in pregnancy 1. For non-pregnant adults, diabetes diagnosis uses either fasting plasma glucose ≥126 mg/dL or a 2-hour value ≥200 mg/dL on a 75-gram oral glucose tolerance test 1.
Women diagnosed with gestational diabetes should undergo postpartum screening for persistent diabetes at 4-12 weeks after delivery using a 75-gram oral glucose tolerance test 1, 2. This is critical because women with gestational diabetes have a 3.4-fold increased risk of developing type 2 diabetes 1.
The two-step approach identifies approximately 5-6% of pregnant women with gestational diabetes, compared to 15-20% with the one-step approach (which uses a 75-gram load without the 3-hour measurement) 1, 2.