A Fasting Oral Glucose Tolerance Test (OGTT – 2 Hour) is a diagnostic blood test that checks how well your body processes sugar (glucose). It measures your blood glucose level after an overnight fast and again 2 hours after you drink a standard glucose (sweet) drink. Because it evaluates your body’s response to a controlled glucose “challenge,” it can detect problems with insulin function that may not show up on a single fasting blood sugar test.

This test is commonly used for diabetes screening—especially gestational diabetes during pregnancy—and can also help diagnose prediabetes or type 2 diabetes.

What the results mean (plain language): Your fasting number shows your baseline blood sugar. The 2-hour number shows how effectively your body moves glucose from the bloodstream into cells after a sugar load. If the 2-hour value is higher than expected, it suggests your body isn’t using insulin efficiently (insulin resistance) or isn’t making enough insulin.

Typical interpretation (non-pregnant adults; exact cutoffs may vary by lab and clinical guidelines):

• Normal glucose tolerance: 2-hour glucose usually <140 mg/dL (7.8 mmol/L)
• Prediabetes (impaired glucose tolerance): 2-hour glucose 140–199 mg/dL (7.8–11.0 mmol/L)
• Diabetes: 2-hour glucose ≥200 mg/dL (11.1 mmol/L) (often confirmed with repeat testing unless symptoms are present)

In pregnancy: OGTT criteria and timing differ (often done between 24–28 weeks or earlier if high risk). Pregnancy-specific cutoffs are used to diagnose gestational diabetes, and your clinician will interpret results according to the testing method ordered (one-step vs two-step approach).

Your clinician may order a 2-hour OGTT when there is concern that your body is not handling glucose normally, or when a more sensitive test is needed than fasting glucose alone. It’s frequently used for glucose tolerance assessment and diabetes screening in both pregnancy and non-pregnant adults.

Common reasons for testing include:

• Pregnancy screening for gestational diabetes, especially if you have risk factors or an abnormal earlier screening result
• Borderline or unclear results on other tests (e.g., mildly elevated fasting glucose or A1C that doesn’t match symptoms)
• Symptoms that may suggest high blood sugar, such as increased thirst, frequent urination, unexplained weight loss, fatigue, blurry vision, slow-healing wounds, or recurrent infections
• Symptoms or history suggesting reactive hypoglycemia (low blood sugar after meals), such as shakiness, sweating, dizziness, or weakness—your clinician may use OGTT patterns alongside symptoms to evaluate this
• Risk factors for type 2 diabetes: overweight/obesity, sedentary lifestyle, age 35+, family history of diabetes, high blood pressure, abnormal cholesterol or triglycerides, history of prediabetes, or having had gestational diabetes previously
• Higher-risk pregnancy factors: prior gestational diabetes, previous baby >9 lb (macrosomia), polycystic ovary syndrome (PCOS), strong family history of diabetes, or certain ethnic backgrounds with increased diabetes risk

How it’s typically performed: You fast (often 8–12 hours), a fasting blood sample is drawn, you drink a measured glucose solution, and your blood is drawn again at 2 hours (sometimes with additional timed samples depending on the protocol). For the most accurate results, follow pre-test instructions about fasting, medications, and activity.

• Gestational diabetes mellitus (GDM)
• Type 2 diabetes mellitus
• Prediabetes (impaired glucose tolerance)
• Insulin resistance / metabolic syndrome
• Polycystic ovary syndrome (PCOS) (often associated with insulin resistance)

• Diabetes screening and early detection of abnormal glucose tolerance
• Pregnancy health planning, including identifying and managing gestational diabetes to reduce risks for parent and baby
• Guiding lifestyle changes (nutrition, weight management, and physical activity) based on glucose tolerance results
• Reducing long-term cardiometabolic risk by identifying insulin resistance early
• Monitoring effectiveness of prevention efforts in people at high risk for type 2 diabetes (as advised by a clinician)