Cutoff Values for Diagnosing Gestational Diabetes Mellitus
Two diagnostic strategies exist for GDM with different cutoff values: the one-step approach using a 75-g OGTT (fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥153 mg/dL with any single value diagnostic) or the two-step approach using a 50-g screening test followed by 100-g OGTT (fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL, 3-hour ≥140 mg/dL with at least two values required). 1
One-Step Strategy (IADPSG Criteria)
The one-step approach uses a 75-g oral glucose tolerance test performed at 24-28 weeks of gestation after an overnight fast of at least 8 hours. 1
Diagnostic cutoffs (any ONE value meets or exceeds):
- 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 15-20% of pregnant individuals with GDM and requires only a single elevated value for diagnosis. 2 These criteria are based directly on pregnancy outcomes from the HAPO study, which demonstrated continuous risk of adverse outcomes without a clear threshold. 1
Two-Step Strategy (Carpenter-Coustan Criteria)
The two-step approach begins with a 50-g glucose load test (GLT) performed at 24-28 weeks, which does not require fasting. 1
Step 1 - Screening cutoffs (proceed to Step 2 if met or exceeded):
- ≥130 mg/dL (7.2 mmol/L) - highest sensitivity (99%) but lower specificity (77%) 1
- ≥135 mg/dL (7.5 mmol/L) - intermediate option 1
- ≥140 mg/dL (7.8 mmol/L) - lower sensitivity (85%) but higher specificity (86%) 1
Step 2 - Diagnostic 100-g OGTT cutoffs (at least TWO values must meet or exceed):
- 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
The two-step approach identifies 5-6% of pregnant individuals with GDM. 2 The American College of Obstetricians and Gynecologists notes that one elevated value can be used for diagnosis, though traditionally two values are required. 1
Early Screening for High-Risk Individuals
High-risk individuals should undergo early glucose testing as soon as feasible after the first prenatal visit using the same diagnostic criteria. 2 High-risk characteristics include: 2
- Marked obesity
- Personal history of GDM
- Glycosuria
- Strong family history of diabetes
- High-risk ethnic populations (Arab, South/Southeast Asian, Latin American)
If early screening is negative, retest at 24-28 weeks. 2
Overt Diabetes in Early Pregnancy
For diagnosing overt diabetes (not GDM) in early pregnancy, use standard diabetes criteria: 2
- Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) 2
- 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during OGTT 2
- Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with symptoms 2
Key Clinical Considerations
The fasting glucose value is the most predictive component of the OGTT. In the two-step approach, fasting values ≥105 mg/dL on the 100-g OGTT have an 81.1% positive predictive value for requiring insulin therapy, compared to 54.0% for values ≥95 mg/dL. 3 This suggests that higher fasting values identify more severe glucose intolerance. 3
The one-step approach identifies twice as many individuals with GDM compared to the two-step approach, but a large randomized trial found no difference in pregnancy and perinatal complications between the two strategies. 1 However, women diagnosed by the one-step approach have a 3.4-fold higher risk of developing prediabetes and type 2 diabetes later in life, suggesting these individuals benefit from long-term diabetes screening. 1, 2
HbA1c is not recommended for GDM screening or diagnosis due to poor test characteristics (sensitivity 82%, specificity 21% at cutoff ≥5.5%). 1 HbA1c should only be used as a secondary measure of glycemic control after diagnosis. 4
The 50-g GLT cutoff of 130 mg/dL captures 90% of GDM cases, while 140 mg/dL captures 80%. 1 The lower threshold increases sensitivity at the cost of more false positives requiring the diagnostic 100-g OGTT. 1