Carpenter-Coustan Criteria for Gestational Diabetes Mellitus Diagnosis
The Carpenter-Coustan criteria are used in the two-step approach for diagnosing gestational diabetes mellitus (GDM), where diagnosis is made 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
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 is a nonfasting test with plasma glucose measurement at 1 hour 1
- If the plasma glucose level measured 1 hour after the load is ≥130,135, or 140 mg/dL (7.2,7.5, or 7.8 mmol/L, respectively), proceed to the diagnostic 100-g OGTT 1
- The threshold of 140 mg/dL provides a sensitivity of 85% and specificity of 78% for detecting GDM 2
- Lower thresholds of 130 mg/dL increase sensitivity to 97% but reduce specificity to 63% 2
Step 2: Diagnostic Test
- The 100-g OGTT should be performed when the patient is fasting (after an overnight fast of at least 8 hours) 1
- Blood samples are collected at fasting and at 1,2, and 3 hours after glucose load 1
- The diagnosis of GDM is made when at least two of the four plasma glucose levels are met or exceeded according to the Carpenter-Coustan criteria 1:
- 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 1, 3:
- Treatment of GDM diagnosed by Carpenter-Coustan criteria has been shown to reduce these adverse outcomes 4
- Women diagnosed with GDM have a markedly increased risk for later development of diabetes, generally type 2 diabetes, after delivery 1
Comparison with Other Diagnostic Criteria
- The Carpenter-Coustan criteria are more sensitive than the older National Diabetes Data Group (NDDG) criteria 5
- The one-step approach using IADPSG criteria (75-g OGTT) diagnoses approximately twice as many women with GDM compared to the two-step approach using Carpenter-Coustan criteria 1
- The IADPSG criteria require only one abnormal value from a 75-g OGTT with different thresholds:
- 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
Clinical Considerations and Pitfalls
- The two-step approach using Carpenter-Coustan criteria is widely used in the United States, while many international organizations have adopted the one-step IADPSG approach 1
- The American Diabetes Association (ADA) now recommends the IADPSG criteria with the intent of optimizing gestational outcomes, as these criteria are based on pregnancy outcomes rather than prediction of subsequent maternal diabetes 1
- However, the two-step approach with Carpenter-Coustan criteria continues to be supported by ACOG 1
- A common pitfall is confusion between screening and diagnostic tests - the 50-g GLT is only a screening test and cannot diagnose GDM even with very high values 2, 6
- Another pitfall is using incorrect thresholds or failing to obtain all four blood samples during the 100-g OGTT 1
The choice between the one-step IADPSG approach and the two-step Carpenter-Coustan approach involves considerations of healthcare resources, as the one-step approach identifies more women requiring treatment but may "medicalize" pregnancies previously considered normal 1.