When should a 50‑gram, 1‑hour glucose challenge test be used instead of a 75‑gram, 2‑hour oral glucose tolerance test in pregnancy?

50g vs 75g Glucose Tolerance Test in Pregnancy

Both the one-step 75g OGTT and the two-step approach (starting with 50g screening) are acceptable strategies for diagnosing gestational diabetes mellitus (GDM), and the choice depends primarily on institutional resources and practice patterns rather than superior outcomes for morbidity and mortality.

Understanding the Two Approaches

The American Diabetes Association recognizes two distinct strategies for GDM screening and diagnosis at 24-28 weeks of gestation 1, 2:

One-Step Strategy (75g OGTT)

• Perform a single 75g oral glucose tolerance test with fasting, 1-hour, and 2-hour plasma glucose measurements 1, 2
• Requires 8-hour overnight fast 1, 2
• Diagnosis made when ANY ONE value meets or exceeds:
  • 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
• Based on IADPSG (International Association of Diabetes and Pregnancy Study Groups) criteria derived from the HAPO study 1

Two-Step Strategy (50g screening followed by 100g OGTT)

• Step 1: 50g glucose load test (GLT) performed non-fasting at 24-28 weeks 1, 2
• If 1-hour plasma glucose ≥130,135, or 140 mg/dL (threshold varies by institution), proceed to Step 2 1
• Step 2: 100g OGTT performed fasting with measurements at 0,1,2, and 3 hours 1, 2
• Diagnosis requires at least TWO elevated values (Carpenter-Coustan criteria):
  • 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

Key Differences in Clinical Impact

Diagnostic Yield

• The one-step approach identifies 2-3 times more women with GDM (18-20% of pregnancies) compared to the two-step approach (5-6% of pregnancies) 3
• This increased detection occurs primarily because only one abnormal value is required for diagnosis rather than two 4, 5, 6

Pregnancy Outcomes

A large randomized controlled trial found no significant differences in composite adverse outcomes (including macrosomia, large-for-gestational-age births, and preeclampsia) between the one-step and two-step approaches when applied to entire pregnancy cohorts 6, 3

However, this trial had important limitations:

• 27% of women randomized to one-step testing underwent two-step testing instead 3
• The study was not powered to detect the likely modest population-level benefits 3
• No long-term maternal or offspring outcomes were assessed 3

Practical Considerations

Advantages of the 50g screening (two-step approach):

• Non-fasting test provides convenience for patients 5, 7
• Only 15-20% of screened women require the diagnostic 100g OGTT 3
• Identifies fewer women as having GDM, requiring less treatment resources 6, 3
• Endorsed by ACOG (American College of Obstetricians and Gynecologists) 1, 6

Advantages of the 75g OGTT (one-step approach):

• Single test visit (no return for diagnostic test) 3
• Shorter duration (2 hours vs 3 hours for 100g OGTT) 3
• Based on pregnancy outcomes from the HAPO study rather than prediction of maternal diabetes 2
• Identifies more women with mild hyperglycemia who may benefit from treatment 4, 5
• 80-90% of women diagnosed can be managed with lifestyle therapy alone 2

When to Use Each Approach

Use the Two-Step Approach (50g screening) When:

• Resource constraints exist for managing increased GDM diagnoses 6, 3
• Patient convenience is prioritized (non-fasting initial screen) 7
• Institutional practice follows ACOG guidelines 1, 6
• Healthcare system capacity for GDM management is limited 3

Use the One-Step Approach (75g OGTT) When:

• Optimizing gestational outcomes is the primary goal 2
• Resources are available to manage the higher number of GDM diagnoses 6, 3
• Patient preference favors a single test despite fasting requirement 3
• Following ADA recommendations for pregnancy outcome-based criteria 2

Critical Caveats

Neither approach should be used for early pregnancy screening (before 24 weeks). The diagnostic criteria were derived from data at 24-28 weeks of gestation and are not validated for earlier screening 1

For high-risk women (marked obesity, prior GDM, strong family history of diabetes), test for pre-existing diabetes at the first prenatal visit using standard non-pregnancy diagnostic criteria, not GDM screening tests 5, 8

The HAPO study demonstrated continuous risk of adverse outcomes with increasing maternal glycemia, with no clear threshold 4, 1. This supports the rationale for the lower diagnostic thresholds in the one-step approach 2

Treatment of mild GDM (as diagnosed by one-step criteria) has been shown to reduce macrosomia and preeclampsia rates in randomized trials, though benefits are modest 2

Bottom Line Algorithm

At 24-28 weeks of gestation in women without known diabetes:

1. If your institution has adopted IADPSG criteria and has resources for increased GDM management: Use the one-step 75g OGTT 2

2. If your institution follows ACOG guidelines or has resource limitations: Use the two-step approach with 50g screening followed by 100g OGTT if positive 1, 6

3. Both approaches are acceptable as neither has demonstrated clear superiority in preventing short-term adverse pregnancy outcomes in head-to-head comparison 6, 3

4. The choice should be based on institutional capacity, guideline adherence, and patient factors rather than expecting major differences in maternal-fetal morbidity or mortality 6, 3