What are the Carpenter criteria for diagnosing diabetes in pregnancy?

Carpenter Criteria for Diagnosis of Diabetes in Pregnancy

The Carpenter-Coustan criteria diagnose gestational diabetes mellitus (GDM) when at least two of the following four plasma glucose values are met or exceeded during a 100-g oral glucose tolerance test (OGTT): 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), and 3-hour 140 mg/dL (7.8 mmol/L). 1, 2

Two-Step Approach Using Carpenter-Coustan Criteria

Step 1: Initial Screening

• Perform a 50-g glucose load test (GLT) at 24-28 weeks of gestation in women not previously diagnosed with diabetes 1
• This test is performed without fasting (non-fasting) 1
• Measure plasma glucose 1 hour after the glucose load 1, 2
• If the 1-hour plasma glucose is ≥130,135, or 140 mg/dL (7.2,7.5, or 7.8 mmol/L), proceed to the diagnostic 100-g OGTT 1
• Different threshold cutoffs have different test characteristics:
  • 140 mg/dL threshold: 85% sensitivity, 86% specificity 1
  • 130 mg/dL threshold: 99% sensitivity, 77% specificity 1

Step 2: Diagnostic Test

• Perform a 100-g OGTT when the patient is fasting (after an overnight fast of at least 8 hours) 1, 2
• Measure plasma glucose at fasting and at 1,2, and 3 hours after glucose load 1
• Diagnosis of GDM is made when at least two of the four plasma glucose levels meet or exceed the Carpenter-Coustan criteria 1, 2:
  • 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)
• The American College of Obstetricians and Gynecologists (ACOG) notes that one elevated value can be used for diagnosis in some cases 1

Clinical Significance and Outcomes

• GDM diagnosed by Carpenter-Coustan criteria is associated with increased risk of adverse maternal and fetal outcomes 2, 3
• Treatment of GDM diagnosed using Carpenter-Coustan criteria has been shown to reduce:
  • Preeclampsia 1
  • Shoulder dystocia 1
  • Macrosomia 1
• Women diagnosed with GDM have significantly increased risk for developing type 2 diabetes later in life 2
• Infants of mothers with GDM diagnosed by Carpenter-Coustan criteria have cord serum insulin levels approximately 40% higher than those of negative screenees 3

Comparison with Other Diagnostic Criteria

• The Carpenter-Coustan criteria are more sensitive than the older National Diabetes Data Group (NDDG) criteria 2, 4
• The Carpenter-Coustan criteria identify approximately 50% more cases of GDM compared to NDDG criteria 3
• The one-step IADPSG approach (75-g OGTT) is more sensitive than Carpenter-Coustan criteria but identifies more women as having GDM (15-20% vs. 5-6%) 1, 2
• The American Diabetes Association (ADA) recommends the IADPSG criteria, while ACOG continues to support the two-step approach with Carpenter-Coustan criteria 1, 2

Clinical Considerations and Pitfalls

• There is no established gold standard for GDM diagnosis, which limits the ability to compare different diagnostic criteria 1
• The choice between one-step IADPSG approach and two-step Carpenter-Coustan approach involves considerations of healthcare resources and potential "medicalization" of pregnancies 2
• In low-risk populations, selective screening based on risk factors may be considered, as less than 1% of women without risk factors develop GDM 5
• When using the 50-g GLT screening test, lowering the threshold from 140 mg/dL to 136 mg/dL increases sensitivity but also increases false positives and healthcare costs 4
• The 100-g OGTT remains the "gold standard" for diagnosing GDM when using the Carpenter-Coustan criteria 6