When should a 75g Oral Glucose Tolerance Test (OGTT) be performed?

When to Perform a 75g Oral Glucose Tolerance Test (OGTT)

Primary Indication: Gestational Diabetes Screening

The 75g OGTT should be performed at 24-28 weeks of gestation in pregnant women not previously diagnosed with diabetes, using the one-step diagnostic approach recommended by the International Association of Diabetes and Pregnancy Study Groups (IADPSG) and adopted by the American Diabetes Association. 1, 2

Testing Protocol Requirements

• Perform the test in the morning after an overnight fast of at least 8 hours 1
• Measure plasma glucose at three time points: fasting, 1 hour, and 2 hours after the 75g glucose load 1, 2
• The diagnosis of gestational diabetes is made when any single value meets or exceeds the following thresholds 1, 2:
  • Fasting: ≥92 mg/dL (5.1 mmol/L)
  • 1 hour: ≥180 mg/dL (10.0 mmol/L)
  • 2 hours: ≥153 mg/dL (8.5 mmol/L)

Alternative Two-Step Approach

While the one-step 75g OGTT is the preferred diagnostic test, some institutions use a two-step approach where the 75g OGTT is not used. Instead, they use a 50g glucose load test followed by a 100g OGTT if positive. 1, 3 The American College of Obstetricians and Gynecologists currently supports the two-step approach, though notes that a single elevated value can be used for diagnosis. 1, 4

Secondary Indication: Diagnosing Diabetes Outside Pregnancy

The 75g OGTT can be used to diagnose diabetes in non-pregnant adults when fasting plasma glucose is <126 mg/dL but clinical suspicion for diabetes remains high due to symptoms compatible with diabetic complications. 5

Diagnostic Criteria for Non-Pregnant Adults

• Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) OR 2-hour post-load glucose ≥200 mg/dL (11.1 mmol/L) indicates diabetes 1
• The test requires an 8-14 hour overnight fast 1
• Impaired glucose tolerance is diagnosed when 2-hour glucose is ≥140 mg/dL (7.8 mmol/L) but <200 mg/dL (11.1 mmol/L) 1

Postpartum Follow-Up

Women diagnosed with gestational diabetes must undergo a 75g OGTT at 4-12 weeks postpartum to screen for persistent diabetes or prediabetes. 2, 4 This population has a 3.4-fold increased risk of developing type 2 diabetes and requires lifelong screening at least every 3 years. 1, 4

Important Clinical Considerations

When NOT to Use the 75g OGTT

• Never use the 75g OGTT to evaluate reactive hypoglycemia - this is an inappropriate indication 5
• Do not perform if overt diabetes is already evident (fasting glucose ≥126 mg/dL or random glucose ≥200 mg/dL with symptoms) 6
• Avoid in patients who cannot tolerate oral glucose loads 1

Confirmation Requirements

• In the absence of unequivocal hyperglycemia or hyperglycemic crisis, abnormal results should be confirmed by repeat testing 1, 6
• When two different tests yield discordant results, repeat the test that exceeded the diagnostic threshold 1, 6

Common Pitfalls to Avoid

• Do not use gestational diabetes thresholds (92/180/153 mg/dL) to diagnose overt diabetes in pregnancy - these are specific to GDM screening only 6
• Ensure proper fasting state - the test must be performed after at least 8 hours of fasting for valid results 1, 6
• Do not rely on a single random glucose measurement without confirmatory testing 6

Prevalence Impact

The one-step 75g OGTT approach identifies approximately 15-20% of pregnant women with gestational diabetes, compared to 5-6% with the two-step approach, representing a 2-3 fold increase in diagnosis. 1, 2, 3 This increase reflects identification of milder hyperglycemia that still carries long-term maternal and offspring metabolic risks. 1