Interpretation of the 75-g OGTT using IADPSG criteria
The one-step
75-g oral glucose tolerance test (OGTT) is interpreted using the
International Association of Diabetes and Pregnancy Study Groups (IADPSG) thresholds. The key principle is that
gestational diabetes mellitus (GDM) is diagnosed when any single plasma glucose value meets or exceeds its threshold—not when two or more values are abnormal. The IADPSG diagnostic cutoffs for the 75-g test are: fasting
92 mg/dL (5.1 mmol/L), 1-hour
180 mg/dL (10.0 mmol/L), and 2-hour
153 mg/dL (8.5 mmol/L).
For this client, the fasting value of
88 mg/dL (4.9 mmol/L) is below the fasting threshold, and the 1-hour value of
176 mg/dL (9.8 mmol/L) is below the 1-hour threshold. However, the 2-hour value of
158 mg/dL (8.8 mmol/L) exceeds the 2-hour threshold of
153 mg/dL (8.5 mmol/L). Because only one elevated value is required for diagnosis under the one-step IADPSG approach, this client has
gestational diabetes.
Watch out! The rule requiring
two or more abnormal values belongs to the
two-step approach, which uses a
100-g, 3-hour OGTT after an initial 50-g glucose challenge. In that protocol, the diagnostic thresholds are higher (fasting 95 mg/dL, 1-hour 180 mg/dL, 2-hour 155 mg/dL, 3-hour 140 mg/dL), and at least two values must be elevated. Confusing the two-step rule with the one-step 75-g test is a common error.
The rationale for using a single elevated value in the one-step approach is grounded in the
Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study data, which demonstrated a continuous, linear relationship between maternal glucose levels and adverse perinatal outcomes—including large-for-gestational-age infants, neonatal hypoglycemia, and cesarean delivery. The IADPSG thresholds were selected to identify pregnancies at meaningfully increased risk, and requiring two abnormal values would miss a substantial proportion of affected pregnancies. The NIH consensus conference noted that the one-step approach diagnoses GDM in approximately
15–18% of pregnancies, whereas the two-step approach yields lower rates, reflecting the different diagnostic philosophies rather than a difference in underlying risk.
A comparison of the two testing strategies clarifies why the interpretation differs:
| Feature | One-step 75-g OGTT (IADPSG) | Two-step 100-g OGTT (Carpenter-Coustan) |
|---|
| Initial step | None; proceed directly to 75-g OGTT after overnight fast | 50-g glucose challenge (non-fasting), then 100-g OGTT if screen positive |
| Diagnostic rule | Any ONE value at or above threshold | TWO or more values at or above threshold |
| Fasting threshold | 92 mg/dL (5.1 mmol/L) | 95 mg/dL (5.3 mmol/L) |
| 1-hour threshold | 180 mg/dL (10.0 mmol/L) | 180 mg/dL (10.0 mmol/L) |
| 2-hour threshold | 153 mg/dL (8.5 mmol/L) | 155 mg/dL (8.6 mmol/L) |
| 3-hour threshold | Not applicable | 140 mg/dL (7.8 mmol/L) |
Key point! The one-step IADPSG approach is more sensitive because it captures glucose intolerance at any single time point. A client with an isolated elevated 2-hour value—as in this case—would be missed by the two-step rule but is correctly identified as having GDM under IADPSG criteria. This has direct implications for nursing care: the client requires immediate referral for medical nutrition therapy, blood glucose self-monitoring, and fetal surveillance, rather than a repeat confirmatory test.
The study comparing 75-g and 100-g OGTTs using IADPSG thresholds found that the 75-g test identified GDM based on single-value criteria, with the 2-hour value frequently contributing to the diagnosis. This supports the clinical observation that post-load hyperglycemia is a common and clinically significant pattern in GDM. The higher diagnostic yield of the one-step approach also means that nurses in high-risk pregnancy units will encounter more clients requiring education on glucose monitoring, dietary modification, and the potential need for insulin therapy when glycemic targets are not met with lifestyle intervention alone.