How is the Oral Glucose Tolerance Test (OGTT) carried out in pregnancy?

How the Oral Glucose Tolerance Test (OGTT) is Performed in Pregnancy

The OGTT in pregnancy is performed at 24-28 weeks of gestation using either a one-step approach (75g glucose load with measurements at fasting, 1-hour, and 2-hour) or a two-step approach (initial 50g screening test followed by 100g diagnostic test if positive), with specific patient preparation and timing requirements that must be strictly followed. 1, 2

Patient Preparation Requirements

Before performing the OGTT, ensure the following conditions are met:

• The test must be done in the morning after an overnight fast of 8-14 hours 1
• The patient should consume at least 150g of carbohydrate per day for at least 3 days before the test 1
• Unlimited physical activity should be maintained in the days leading up to the test 1
• The patient must remain seated throughout the entire test and should not smoke 1

One-Step Approach (IADPSG Criteria)

This approach uses a 75g glucose load with three measurement time points 1, 2:

Measurement Schedule:

• Fasting (baseline)
• 1 hour after glucose administration
• 2 hours after glucose administration 1, 2

Diagnostic Thresholds (any single value meeting or exceeding these indicates GDM):

• 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) 1, 2

This approach identifies approximately 15-20% of pregnant women with GDM, which is 2-3 times more cases than the two-step approach. 2

Two-Step Approach (ACOG-Supported)

Step 1: Initial Screening (50g Glucose Challenge Test)

• Performed at 24-28 weeks of gestation 1, 2
• Does not require fasting 1, 2
• Single blood draw at 1 hour after 50g glucose load 1
• Proceed to Step 2 if glucose ≥140 mg/dL (some use ≥130 mg/dL threshold) 1

Step 2: Diagnostic Test (100g OGTT)

Performed only if Step 1 is positive, requires fasting 1

Measurement Schedule:

• Fasting (baseline)
• 1 hour after 100g glucose administration
• 2 hours after glucose administration
• 3 hours after glucose administration 1

Diagnostic Thresholds (Carpenter/Coustan criteria - at least 2 of 4 values must meet or exceed):

• 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) 1, 2

This approach identifies approximately 5-6% of pregnant women with GDM. 2

Special Circumstances: 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 (12-14 weeks). 1, 3

• If negative at first prenatal visit, these high-risk women must be retested at 24-28 weeks of gestation 1, 3
• Early testing uses the same OGTT protocols but aims to detect pre-existing undiagnosed type 2 diabetes 3

Important Clinical Caveats

The analytical reliability of the OGTT has limitations - research shows that with the maximum allowable analytical error of 6.9% for glucose measurements, GDM prevalence could theoretically vary from 4.5% to 25.3% depending on measurement variability, compared to the standard 12.2% prevalence. 4 This underscores the importance of proper laboratory technique and quality control.

ACOG currently supports the two-step approach but notes that a single elevated value can be used for GDM diagnosis, representing an evolution in diagnostic thinking. 2

The choice between one-step and two-step approaches remains controversial and depends on factors including GDM prevalence in your population, available resources, and cost-effectiveness considerations. 2 The one-step approach may be more cost-effective in high-risk populations. 1