What are the diagnostic methods for Gestational Diabetes Mellitus (GDM) according to different guidelines?

Diagnostic Methods for Gestational Diabetes Mellitus (GDM) According to Different Guidelines

The diagnosis of gestational diabetes mellitus (GDM) can be accomplished using either the one-step 75g OGTT approach or the two-step approach with an initial 50g glucose screening test followed by a 100g OGTT for those who screen positive. 1

Timing of Screening

• All pregnant women not previously known to have diabetes should be screened for GDM at 24-28 weeks of gestation 1, 2
• Women with risk factors (obesity, previous GDM, family history of diabetes, high-risk ethnicity) should be tested for undiagnosed type 2 diabetes at the first prenatal visit 1, 2, 3
• If early screening is negative in high-risk women, they should be retested at 24-28 weeks 3

One-Step Strategy (IADPSG Criteria)

• Perform a 75g oral glucose tolerance test (OGTT) at 24-28 weeks of gestation 1, 4
• The OGTT should be performed in the morning after an overnight fast of at least 8 hours 1
• Measure plasma glucose when fasting and at 1 and 2 hours after glucose load 1
• GDM is diagnosed when any of the following plasma glucose values are met or exceeded:
  • Fasting: ≥92 mg/dL (5.1 mmol/L) 1
  • 1 hour: ≥180 mg/dL (10.0 mmol/L) 1
  • 2 hour: ≥153 mg/dL (8.5 mmol/L) 1
• This approach identifies more women with GDM (15-20%) compared to the two-step approach (5-6%) 3, 5
• The American Diabetes Association (ADA) recommends these criteria as they are the only ones based on pregnancy outcomes rather than endpoints such as prediction of subsequent maternal diabetes 1

Two-Step Strategy

• Step 1: Perform a 50g glucose load test (GLT) at 24-28 weeks of gestation (nonfasting) with plasma glucose measurement at 1 hour 1
• If the plasma glucose level is ≥130,135, or 140 mg/dL (7.2,7.5, or 7.8 mmol/L), proceed to a 100g OGTT 1
  • The American College of Obstetricians and Gynecologists (ACOG) recommends any of these commonly used thresholds 1, 2
  • A lower threshold of 135 mg/dL (7.5 mmol/L) is recommended for high-risk ethnic populations 1
• Step 2: Perform a 100g OGTT when the patient is fasting 1
• GDM is diagnosed when 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
  • 1 hour: ≥180 mg/dL (10.0 mmol/L) 1
  • 2 hour: ≥155 mg/dL (8.6 mmol/L) 1
  • 3 hour: ≥140 mg/dL (7.8 mmol/L) 1
• ACOG notes that one elevated value can be used for diagnosis 1, 2
• The ACOG currently supports this two-step approach 2, 6

Diagnostic Criteria Variations Between Guidelines

• The National Diabetes Data Group (NDDG) criteria for the two-step approach has higher thresholds:
  • Fasting: ≥105 mg/dL (5.8 mmol/L) 1
  • 1 hour: ≥190 mg/dL (10.6 mmol/L) 1
  • 2 hour: ≥165 mg/dL (9.2 mmol/L) 1
  • 3 hour: ≥145 mg/dL (8.0 mmol/L) 1
• In Germany, a 1-hour 50g oral glucose challenge test is followed by a 2-hour 75g OGTT if the first test is pathological 7
• The WHO criteria are based on the HAPO study and align with the IADPSG one-step approach 8, 9

Postpartum Follow-up

• Women with GDM should be screened for persistent diabetes at 4-12 weeks postpartum using a 75g OGTT and clinically appropriate non-pregnancy diagnostic criteria 1, 4
• 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, 4

Clinical Considerations and Controversies

• The one-step approach identifies more cases of GDM but its clinical benefit in reducing adverse pregnancy outcomes remains controversial 5, 8
• Women diagnosed with GDM by the one-step approach have a 3.4-fold higher risk of developing prediabetes and type 2 diabetes later in life 4, 3
• Different glucose response patterns in the 75g OGTT are associated with different adverse perinatal outcomes, suggesting an individualized approach to prenatal care after GDM diagnosis 7
• The anticipated increase in GDM diagnosis with the one-step approach could impact healthcare costs and medical infrastructure needs 1, 8