What is the test used to diagnose gestational diabetes?

Diagnostic Tests for Gestational Diabetes

The standard test for diagnosing gestational diabetes is the oral glucose tolerance test (OGTT), with initial screening typically performed using the 50-gram glucose challenge test (Glucola test) followed by a diagnostic OGTT if the screening is abnormal. 1, 2

Screening Process

Two-Step Approach (Most Common in the US)

1. Initial Screening: 50-gram Glucose Challenge Test (Glucola test)

   • Performed at 24-28 weeks gestation
   • Non-fasting test (can be done at any time of day)
   • Blood glucose measured 1 hour after consuming 50g glucose solution
   • Abnormal result: ≥140 mg/dL (80% sensitivity) or ≥130 mg/dL (90% sensitivity) 2, 1

2. Diagnostic Test: 100-gram OGTT

   • Performed if screening test is abnormal
   • Requires fasting overnight (at least 8 hours)
   • Blood glucose measured at fasting, 1-hour, 2-hour, and 3-hour after consuming 100g glucose
   • GDM diagnosed when two or more values meet or exceed thresholds:
     • Fasting: ≥95 mg/dL
     • 1-hour: ≥180 mg/dL
     • 2-hour: ≥155 mg/dL
     • 3-hour: ≥140 mg/dL 2

One-Step Approach (IADPSG/WHO Criteria)

• 75-gram OGTT
  • Performed directly at 24-28 weeks gestation
  • Requires fasting
  • Blood glucose measured at fasting, 1-hour, and 2-hour after consuming 75g glucose
  • GDM diagnosed when any one value meets or exceeds thresholds:
    • Fasting: ≥92 mg/dL
    • 1-hour: ≥180 mg/dL
    • 2-hour: ≥153 mg/dL 1, 2

Timing of Testing

• All pregnant women not previously known to have diabetes should be screened at 24-28 weeks gestation 1, 2
• Women with high-risk factors should be screened at their first prenatal visit 1, 2
• High-risk factors include:
  • Marked obesity
  • Personal history of gestational diabetes
  • Strong family history of diabetes
  • Previous macrosomic infant (>4500g)
  • History of adverse pregnancy outcomes 2

Clinical Considerations

• The two-step approach identifies fewer women with GDM (5-6%) compared to the one-step approach (15-20%) 1, 3
• The USPSTF previously stated there was insufficient evidence to recommend for or against screening, but more recent guidelines support universal screening at 24-28 weeks 1, 2
• Treatment of gestational diabetes reduces the risk of preeclampsia, macrosomia, and shoulder dystocia 1, 2
• Different glucose patterns during OGTT may predict different adverse outcomes:
  • Isolated fasting hyperglycemia is associated with higher birthweight and LGA infants
  • Isolated post-load hyperglycemia is associated with SGA infants 4

Post-Pregnancy Follow-up

• Women with GDM should be screened for persistent diabetes at 4-12 weeks postpartum using a 75g OGTT 1, 2
• Lifelong screening for diabetes should occur at least every 3 years 1, 2
• Women with GDM have a 50-60% lifetime risk of developing type 2 diabetes 2

Common Pitfalls

• Using HbA1c for postpartum screening (OGTT is preferred) 2
• Failing to perform postpartum screening (only 30-50% of women receive appropriate follow-up) 2
• Using inappropriate diagnostic thresholds or incorrect glucose load 2
• Discontinuing long-term surveillance after initial normal postpartum screening 2
• Missing high-risk women who need early screening in the first trimester 2

The choice between one-step and two-step approaches remains controversial, with ongoing debate about the balance between identifying more at-risk women versus increased healthcare costs and potential medicalization of pregnancy 3.