Oral Glucose Tolerance Test: Procedure and Diagnostic Criteria
Non-Pregnant Adults
For non-pregnant adults, use the fasting plasma glucose test as the preferred diagnostic method, reserving the 75-gram OGTT for patients with fasting glucose <126 mg/dL who have symptoms suggestive of diabetes complications. 1
Test Preparation
- Patients must consume at least 150 grams of carbohydrate daily for 3 consecutive days before testing 2
- Maintain unlimited physical activity in the days preceding the test 2
- Fast for 8-14 hours overnight before the test 2
- The test must be performed in the morning 2
- Patients should remain seated throughout the test and not smoke 2
Test Administration
- Administer 75 grams of anhydrous glucose dissolved in water 2
- Measure venous plasma glucose at fasting, 1-hour, and 2-hour time points 2
Diagnostic Thresholds (Non-Pregnant)
- Fasting plasma glucose ≥126 mg/dL 2
- 2-hour plasma glucose ≥200 mg/dL 2
- Confirm diagnosis by repeat testing on a different day unless unequivocal hyperglycemia is present 2
Important Caveat
The OGTT is poorly reproducible and difficult to perform in routine practice, making fasting plasma glucose the preferred test for most non-pregnant adults 1. The OGTT should never be used to evaluate reactive hypoglycemia 3.
Pregnant Women
All pregnant women without pre-existing diabetes must be screened for gestational diabetes mellitus (GDM) at 24-28 weeks gestation using either a one-step 75-gram OGTT or a two-step approach. 4
Timing of Screening
Standard Risk Women
- Screen at 24-28 weeks gestation 4
- This window corresponds to peak pregnancy-related insulin resistance 4
- Do not postpone screening beyond 28 weeks—this timing is critical for timely intervention to reduce adverse maternal-fetal outcomes 4
High-Risk Women (Early Screening)
Perform glucose testing at the first prenatal visit (12-14 weeks) if any of the following are present: 4
If early testing is negative, retest at 24-28 weeks 4
Low-Risk Women (May Omit Screening)
Women meeting all of the following criteria may forgo GDM screening: 4
- Age <25 years
- Pre-pregnancy BMI ≤25 kg/m²
- No first-degree relative with diabetes
- No prior abnormal glucose tolerance
- No history of adverse obstetric outcomes
- Ethnic group with low diabetes prevalence
Two Screening Approaches for Pregnancy
One-Step Approach (IADPSG/ADA Recommended)
The one-step 75-gram OGTT is the preferred method, identifying approximately 15-20% of pregnancies with GDM. 4
Test Preparation
- 8-14 hour overnight fast 4
- ≥150 grams carbohydrate daily for 3 days before testing 4
- Patient must remain seated and not smoke during the test 4
Test Administration
- Administer 75 grams glucose after overnight fast 4
- Measure plasma glucose at fasting, 1-hour, and 2-hour 4
Diagnostic Thresholds (One-Step)
GDM is diagnosed when ANY ONE value meets or exceeds: 1, 4
These thresholds are based on the HAPO study, which demonstrated continuous, graded increases in adverse outcomes (macrosomia, cesarean delivery, neonatal hypoglycemia, shoulder dystocia) as maternal glucose rises, without a clear safety threshold 1.
Two-Step Approach (ACOG-Supported)
The two-step approach identifies approximately 5-6% of pregnancies with GDM, missing milder cases but requiring fewer women to undergo full diagnostic testing. 4
Step 1: Initial Screening
- Administer 50-gram glucose challenge (non-fasting) at 24-28 weeks 4
- Measure plasma glucose at 1 hour 4
- If 1-hour glucose is ≥130,135, or 140 mg/dL (threshold varies by institution), proceed to Step 2 4
Step 2: Diagnostic Test
- Perform 100-gram OGTT after overnight fast 4
- Measure plasma glucose at fasting, 1-hour, 2-hour, and 3-hour 4
Diagnostic Thresholds (Two-Step, Carpenter-Coustan Criteria)
GDM is diagnosed when AT LEAST TWO values meet or exceed: 4, 5
- Fasting ≥95 mg/dL (5.3 mmol/L) 5
- 1-hour ≥180 mg/dL (10.0 mmol/L) 5
- 2-hour ≥155 mg/dL (8.6 mmol/L) 5
- 3-hour ≥140 mg/dL (7.8 mmol/L) 5
Important note: ACOG acknowledges that in clinical practice, a single elevated value may be used for diagnosis, though traditional criteria require two abnormal values 4, 5.
Critical Pitfalls to Avoid
Do Not Use Fasting Glucose Alone at 24-28 Weeks
Using only fasting plasma glucose at 24-28 weeks is NOT validated and results in massive under-diagnosis of GDM. 4 The 1-hour and 2-hour post-load values independently predict macrosomia and adverse outcomes 4. A complete OGTT with all three time points is mandatory 4.
Early Pregnancy Exception
Fasting glucose alone may be used only at the first prenatal visit (early pregnancy) in high-risk women: 4
- Fasting glucose ≥92 mg/dL but <126 mg/dL diagnoses GDM
- Fasting glucose ≥126 mg/dL indicates overt diabetes (not GDM)
These early-pregnancy thresholds do not apply to standard 24-28 week screening 4.
Do Not Use Urine Glucose Testing
Urine glucose testing is not useful for GDM screening or management and should be avoided 4.
Do Not Apply IADPSG Thresholds Before 24 Weeks
The IADPSG diagnostic thresholds were derived from data collected at 24-32 weeks and should not be applied earlier in pregnancy 4.
Choosing Between One-Step and Two-Step
In high-risk populations, the one-step strategy is more cost-effective and identifies a larger number of women with milder hyperglycemia who still benefit from treatment. 4 The two-step approach reduces the number of women requiring full diagnostic testing but may miss milder cases of GDM 4. Both strategies are acceptable; the choice depends on practice setting, GDM prevalence in the population, and available resources 4.
Postpartum Follow-Up
All women diagnosed with GDM must undergo a 75-gram OGTT at 4-12 weeks postpartum using non-pregnancy diagnostic criteria to detect persistent diabetes or prediabetes 5. Women with prior GDM have a 3.4-fold increased risk of developing type 2 diabetes and require lifelong diabetes screening every 2-3 years 5.