What are the diagnostic criteria for Gestational Diabetes Mellitus (GDM)?

Diagnostic Criteria for Gestational Diabetes Mellitus (GDM)

There are two accepted approaches for diagnosing GDM: the one-step 75g OGTT and the two-step approach with initial 50g screening followed by 100g OGTT, both performed at 24-28 weeks of gestation in pregnant women not previously known to have diabetes. 1, 2

One-Step Strategy (75g OGTT)

• Perform a 75g oral glucose tolerance test (OGTT) at 24-28 weeks of gestation 1, 2
• The OGTT should be performed in the morning after an overnight fast of at least 8 hours 1, 3
• Measure plasma glucose when fasting and at 1 and 2 hours after glucose load 1, 2
• GDM is diagnosed when ANY ONE of the following plasma glucose values is met or exceeded:
  • Fasting: ≥92 mg/dL (5.1 mmol/L) 1, 2
  • 1 hour: ≥180 mg/dL (10.0 mmol/L) 1, 2
  • 2 hours: ≥153 mg/dL (8.5 mmol/L) 1, 2

Two-Step Strategy

Step 1:

• Perform a 50g glucose load test (GLT) at 24-28 weeks of gestation 1
• No fasting is required 1, 3
• Measure plasma glucose at 1 hour after the load 1
• Proceed to 100g OGTT if plasma glucose is ≥130-140 mg/dL (7.2-7.8 mmol/L) 1
  • Threshold options: ≥130 mg/dL, ≥135 mg/dL, or ≥140 mg/dL 1
  • ACOG recommends a lower threshold of 135 mg/dL in high-risk ethnic populations 1

Step 2:

• Perform a 100g OGTT when the patient is fasting 1
• Measure plasma glucose when fasting and at 1,2, and 3 hours after glucose load 1
• GDM is diagnosed when at least two of the following plasma glucose values are met or exceeded (Carpenter-Coustan criteria):
  • Fasting: ≥95 mg/dL (5.3 mmol/L) 1, 2
  • 1 hour: ≥180 mg/dL (10.0 mmol/L) 1, 2
  • 2 hours: ≥155 mg/dL (8.6 mmol/L) 1, 2
  • 3 hours: ≥140 mg/dL (7.8 mmol/L) 1, 2
• ACOG notes that one elevated value can be used for diagnosis 1, 2

Early Screening for High-Risk Women

• Test for undiagnosed diabetes at the first prenatal visit in women with risk factors 1, 4
• Risk factors include:
  • Marked obesity
  • Personal history of GDM
  • Glycosuria
  • Strong family history of diabetes
  • Membership in high-risk ethnic populations 4
• Use standard diagnostic criteria for early screening 4
• If early screening is negative, high-risk women should be retested at 24-28 weeks 4

Diagnostic Criteria for Overt Diabetes in Early Pregnancy

• Fasting plasma glucose ≥126 mg/dL 4
• 2-hour plasma glucose ≥200 mg/dL during OGTT 4
• Random plasma glucose ≥200 mg/dL with symptoms of hyperglycemia 4

Clinical Considerations

• The one-step approach identifies significantly more women with GDM (15-20%) compared to the two-step approach (5-6%) 4, 5, 6
• The American College of Obstetricians and Gynecologists (ACOG) currently supports the two-step approach 2, 3
• The one-step method identifies women at long-term risk of maternal prediabetes and diabetes 2, 4
• Women diagnosed by the one-step approach have a 3.4-fold higher risk of developing prediabetes and type 2 diabetes later in life 4
• Despite identifying more cases, some studies show no significant difference in perinatal outcomes between the two approaches 7, 6

Post-GDM Follow-up

• Test women with GDM for persistent diabetes at 4-12 weeks postpartum using a 75g OGTT 1, 2
• Women with a history of GDM should have lifelong screening for diabetes or prediabetes at least every 3 years 1, 2
• Women with a history of GDM found to have prediabetes should receive intensive lifestyle interventions or metformin to prevent diabetes 1