Oral Glucose Tolerance Test (OGTT): Purpose, Procedure & What It Reveals

The Oral Glucose Tolerance Test (OGTT) evaluates how efficiently your body processes glucose. It is a key diagnostic tool for detecting insulin resistance, prediabetes, type 2 diabetes, and hypoglycemia (low blood sugar).

How the Test Works

After an overnight fast, a baseline blood sample is taken. You then drink a standardized glucose solution (usually 75 g). Blood glucose is measured at intervals—typically 1 hour and 2 hours post‑drink—to track how quickly levels rise and fall.

What the Results Indicate

• Normal: Fasting <100 mg/dL; 2‑hour <140 mg/dL
• Impaired fasting glucose / prediabetes: Fasting 100‑125 mg/dL or 2‑hour 140‑199 mg/dL
• Diabetes: Fasting ≥126 mg/dL or 2‑hour ≥200 mg/dL
• Reactive hypoglycemia: A sharp drop in glucose below 70 mg/dL after the peak

When It’s Ordered

Doctors may request an OGTT if fasting glucose is borderline, during pregnancy to screen for gestational diabetes, or when symptoms suggest blood‑sugar dysregulation despite normal fasting numbers.

Always follow your provider’s preparation instructions (fasting, medication adjustments) for accurate results.