Sensitivity of OGTT for Gestational Diabetes Mellitus
The sensitivity of the oral glucose tolerance test (OGTT) for diagnosing gestational diabetes mellitus depends critically on which screening approach and glucose threshold you use: the 50-g glucose challenge test (OGCT) with a 130 mg/dL cutoff achieves 99% sensitivity, while the 140 mg/dL cutoff achieves 85% sensitivity when confirmed by the diagnostic 100-g OGTT. 1
Two-Step Approach Sensitivity
The two-step screening strategy demonstrates variable sensitivity based on the initial screening threshold:
- 50-g OGCT at 130 mg/dL cutoff: 99% sensitivity with 77% specificity when GDM is confirmed using Carpenter-Coustan criteria on the subsequent 100-g OGTT 1
- 50-g OGCT at 140 mg/dL cutoff: 85% sensitivity with 86% specificity using the same confirmation criteria 1
The lower threshold (130 mg/dL) captures nearly all cases of GDM but requires more women to undergo the confirmatory 100-g test, while the higher threshold (140 mg/dL) misses approximately 15% of GDM cases but has better specificity. 1
One-Step Approach Sensitivity
The one-step approach using the 75-g OGTT with IADPSG criteria (fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, or 2-hours ≥153 mg/dL) identifies approximately 2-3 times more women with GDM compared to the two-step method, with prevalence rates of 15-20% versus 5-6%. 2 This suggests the one-step method has higher sensitivity for detecting milder degrees of glucose intolerance, though this comes at the cost of diagnosing and treating more women. 1, 2
Alternative Screening Tests Performance
For comparison with other screening modalities:
- Fasting plasma glucose at 85 mg/dL: 87% sensitivity, 52% specificity 1
- Fasting plasma glucose at 90 mg/dL: 77% sensitivity, 76% specificity 1
- Fasting plasma glucose at 95 mg/dL: 54% sensitivity, 93% specificity 1
- HbA1c at 5.5%: 82% sensitivity, 21% specificity 1
The OGCT outperforms both fasting plasma glucose and HbA1c for identifying women with abnormal glucose responses to larger glucose loads. 1
Critical Clinical Considerations
A major limitation in interpreting OGTT sensitivity is the lack of an established gold standard for GDM diagnosis, as different diagnostic criteria (Carpenter-Coustan, WHO, IADPSG, NDDG) identify different populations and degrees of maternal hyperglycemia. 1 This makes direct comparison across studies challenging and explains why sensitivity estimates vary based on which reference standard is used.
The choice of screening threshold involves a trade-off between sensitivity and specificity: using the 130 mg/dL cutoff for the 50-g OGCT maximizes case detection (99% sensitivity) but requires more confirmatory testing, while the 140 mg/dL cutoff balances sensitivity (85%) with specificity (86%) and reduces the number of women requiring the full 100-g OGTT. 1
Early pregnancy OGTT (before 24 weeks) has insufficient sensitivity to reliably predict or exclude GDM that develops later in pregnancy, with research showing low sensitivity for identifying women who will develop GDM or deliver large-for-gestational-age babies. 3, 4 Standard screening should occur at 24-28 weeks of gestation when the sensitivity is optimized. 1