What are the diagnostic glucose thresholds for a pregnancy oral glucose tolerance test and the recommended management for gestational diabetes diagnosed by these criteria?

Interpreting Oral Glucose Tolerance Test Results in Pregnancy

Diagnostic Approach: Two Accepted Strategies

You should use either the one-step approach (75g OGTT with single abnormal value diagnostic) or the two-step approach (50g screening followed by 100g OGTT if positive), with the one-step method being more sensitive and identifying approximately 15-20% of pregnancies versus 5-6% with the two-step method. 1, 2

One-Step Approach (IADPSG/ADA Criteria)

Perform a 75g OGTT at 24-28 weeks of gestation with plasma glucose measurements at fasting, 1-hour, and 2-hour time points. 1, 2, 3

Diagnostic thresholds - GDM is diagnosed if ANY ONE of the following values is met or exceeded: 1, 2, 3

• 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)

This approach requires only a single abnormal value for diagnosis, making it more inclusive and identifying milder degrees of hyperglycemia that still carry increased risk for adverse outcomes. 1 The thresholds were derived from the HAPO study, which demonstrated continuous increases in adverse maternal and fetal outcomes with rising glucose levels, even within previously "normal" ranges. 1

Two-Step Approach (ACOG-Supported)

Step 1: Perform a 50g glucose challenge test (non-fasting) at 24-28 weeks. 1, 2

• If plasma glucose at 1-hour is ≥130,135, or 140 mg/dL (7.2,7.5, or 7.8 mmol/L), proceed to Step 2. 1

Step 2: Perform a 100g OGTT (fasting) with measurements at fasting, 1-hour, 2-hour, and 3-hour. 1, 2

Diagnostic thresholds (Carpenter-Coustan criteria) - GDM is diagnosed if AT LEAST TWO of the following values are met or exceeded: 1, 2

• 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)

Important caveat: ACOG notes that a single elevated value can be used for diagnosis in clinical practice, though the traditional criteria require two abnormal values. 1, 4

Test Preparation Requirements

The OGTT must be performed in the morning after an overnight fast of 8-14 hours. 1, 2, 3

The patient must consume at least 150g of carbohydrate per day for at least 3 days before the test. 1, 2

The patient should maintain unlimited physical activity in the days leading up to the test, remain seated throughout the entire test, and not smoke during testing. 1, 2

Special Populations: High-Risk Women

Women with marked obesity (BMI ≥30 kg/m²), personal history of GDM, glycosuria, or strong family history of diabetes should undergo glucose testing as early as the first prenatal visit. 1, 2

If testing is negative at the first prenatal visit, these high-risk women must be retested at 24-28 weeks of gestation. 1, 2

If fasting plasma glucose in early pregnancy is ≥5.1 mmol/L (92 mg/dL) but <7.0 mmol/L (126 mg/dL), diagnose as GDM. 1

If fasting plasma glucose is ≥7.0 mmol/L (126 mg/dL) at any point, this indicates overt diabetes, not GDM, and requires treatment as preexisting diabetes. 1, 3

Management After Diagnosis

Initiate blood glucose self-monitoring at least 4 times daily: fasting upon waking and 1-hour postprandial after each meal (breakfast, lunch, dinner). 4

Target glucose levels to avoid pharmacologic therapy: 4

• Fasting <95 mg/dL
• 1-hour postprandial <140 mg/dL
• 2-hour postprandial <120 mg/dL

If glycemic targets are not achieved within 1-2 weeks of lifestyle modifications (medical nutrition therapy), add insulin therapy, which is the preferred first-line pharmacologic treatment during pregnancy. 4

Initiate ultrasound surveillance to assess fetal abdominal circumference, with measurements exceeding the 75th percentile for gestational age indicating fetal hyperinsulinemia and requiring more intensive glycemic control. 1, 4

Critical Pitfalls to Avoid

Do not use the IADPSG or two-step diagnostic criteria for testing before 24 weeks of gestation, as these thresholds were not derived from data in the first half of pregnancy. 1

Do not rely on urine glucose monitoring for GDM management, as it is not useful. 1

Ensure postpartum follow-up: Screen for persistent diabetes at 4-12 weeks postpartum using a 75g OGTT, as women with GDM have a 3.4-fold increased risk of developing type 2 diabetes and require lifelong screening at least every 3 years. 4

Choosing Between Approaches

The one-step approach is more cost-effective in high-risk populations and identifies more women with milder hyperglycemia who still benefit from treatment. 1 The two-step approach requires fewer women to undergo the full diagnostic OGTT but may miss cases of milder GDM. 1, 2 Both approaches are acceptable, and the choice depends on your practice setting, patient population prevalence of GDM, and available resources. 2