What is the recommended process for diagnosing gestational diabetes?

Diagnosing Gestational Diabetes

The recommended process for diagnosing gestational diabetes involves screening all pregnant women at 24-28 weeks of gestation using either the one-step 75g OGTT or the two-step approach with initial 50g glucose challenge test followed by a 100g OGTT if positive. 1, 2

Timing of Screening

• All pregnant women not previously known to have diabetes should be screened for gestational diabetes mellitus (GDM) at 24-28 weeks of gestation 1, 2
• Women with risk factors should be tested for undiagnosed type 2 diabetes at the first prenatal visit 1
• Risk factors warranting early screening include:
  • BMI ≥30 kg/m² 3
  • History of previous GDM 3
  • Family history of diabetes in first-degree relatives 3
  • High-risk ethnicity (Hispanic, Native American, South or East Asian, African American, Pacific Islander) 3
  • History of delivering a macrosomic baby (>4.05 kg) 3
  • History of polycystic ovary syndrome 3

Diagnostic Approaches

One-Step Strategy (75g OGTT)

• Perform a 75g oral glucose tolerance test (OGTT) with plasma glucose measurements when patient is fasting and at 1 and 2 hours 1, 2
• The test should be performed in the morning after an overnight fast of at least 8 hours 1, 2
• Diagnosis of GDM is made when any of the following plasma glucose values are met or exceeded:
  • 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 2-3 times more cases of GDM than the two-step approach, with a prevalence of 15-20% versus 5-6% 2

Two-Step Strategy

• Step 1: Perform a 50g glucose load test (GLT) (nonfasting), with plasma glucose measurement at 1 hour 1, 2
• If the plasma glucose level is ≥130-140 mg/dL (7.2-7.8 mmol/L), proceed to a 100g OGTT 1, 2
• Step 2: The 100g OGTT should be performed when the patient is fasting 1, 2
• Diagnosis of GDM is made if at least two of the following four plasma glucose levels are met or exceeded:
  • 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
• The American College of Obstetricians and Gynecologists (ACOG) currently supports this approach 2

Controversies and Considerations

• There is ongoing debate between the one-step and two-step approaches 1, 4
• The one-step approach is more sensitive but may lead to over-diagnosis of GDM 5
• The two-step approach is more practical as the initial 50g test does not require fasting 2
• The choice between approaches may depend on factors such as population prevalence, available resources, and cost-effectiveness 2, 4

Post-Diagnosis Follow-up

• Women with GDM should be screened for persistent diabetes at 4-12 weeks postpartum using a 75g OGTT 1, 3, 6
• Women with a history of GDM should have lifelong screening for diabetes or prediabetes at least every 3 years 1, 3
• Women found to have prediabetes should receive intensive lifestyle interventions or metformin to prevent diabetes 1, 3

Common Pitfalls to Avoid

• Failing to screen high-risk women early in pregnancy 3
• Not repeating screening at 24-28 weeks in high-risk women who initially test negative 3
• Overlooking the importance of postpartum screening for persistent diabetes 6
• Using incorrect diagnostic thresholds or testing protocols 4

The diagnostic approach for gestational diabetes continues to evolve, with ongoing research aimed at establishing international consensus on the optimal screening and diagnostic criteria to improve maternal and fetal outcomes 4, 5.