What defines an abnormal 3‑hour oral glucose tolerance test (OGTT)?

Abnormal 3-Hour Glucose Tolerance Test

An abnormal 3-hour glucose tolerance test (100-gram OGTT) is diagnosed when at least two of the four plasma glucose values meet or exceed the following thresholds: 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), and 3-hour ≥140 mg/dL (7.8 mmol/L), using the Carpenter-Coustan criteria. 1

Diagnostic Criteria

The 3-hour OGTT is part of the two-step approach for diagnosing gestational diabetes mellitus (GDM):

Step 1: Initial Screening

• Perform a 50-gram glucose loading test (GLT) at 24-28 weeks of gestation (non-fasting) 1
• If plasma glucose at 1 hour is ≥130,135, or 140 mg/dL (7.2,7.5, or 7.8 mmol/L), proceed to the 3-hour OGTT 1

Step 2: Diagnostic Testing

• The 100-gram OGTT must be performed after an overnight fast of at least 8 hours 1
• Blood samples are drawn at four time points: fasting, 1 hour, 2 hours, and 3 hours after glucose administration 1, 2

Diagnostic Thresholds (Carpenter-Coustan Criteria)

The test is abnormal when at least two of these four values are met or exceeded: 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)

Important Clinical Considerations

Single Abnormal Value

The American College of Obstetricians and Gynecologists notes that one elevated value can be used for diagnosis, though the traditional criterion requires at least two abnormal values. 1 This is clinically significant because women with only one abnormal value on the 3-hour OGTT have substantially increased risks compared to those with normal results: 3

• Macrosomia: 1.59-fold increased risk
• Large for gestational age: 1.38-fold increased risk
• Neonatal hypoglycemia: 1.88-fold increased risk
• Cesarean delivery: 1.69-fold increased risk
• Pregnancy-induced hypertension: 1.55-fold increased risk

These outcomes are comparable to women diagnosed with frank GDM, suggesting that even a single abnormal value warrants clinical attention. 3

Alternative Criteria

The older National Diabetes Data Group (NDDG) criteria use slightly higher thresholds: fasting ≥105 mg/dL, 1-hour ≥190 mg/dL, 2-hour ≥165 mg/dL, and 3-hour ≥145 mg/dL. 1 However, the Carpenter-Coustan criteria are more commonly used and identify more women at risk. 1

Common Pitfalls and How to Avoid Them

Ensure Proper Test Preparation

• The patient must fast for at least 8 hours before the test 1
• The test should be performed in the morning 1
• Failure to follow these protocols can lead to inaccurate results

Don't Dismiss Borderline Results

• Women with one abnormal value have significantly worse pregnancy outcomes than those with completely normal tests 3
• Consider dietary counseling and glucose monitoring even for single abnormal values, as treatment can reduce macrosomia rates 4

Postpartum Follow-Up is Essential

• All women diagnosed with GDM must undergo a 75-gram OGTT at 4-12 weeks postpartum to assess for persistent diabetes 1, 2
• Women with GDM have a 3.4-fold increased risk of developing type 2 diabetes later in life 1, 5

Prevalence and Clinical Impact

The two-step approach identifies approximately 5-6% of pregnant women with GDM, compared to 15-20% with the one-step approach using a 75-gram OGTT. 1, 5 The choice between approaches remains controversial, but the two-step method is currently supported by the American College of Obstetricians and Gynecologists. 1, 2