Quick Facts
- Sample: Venous blood (4 specimens)
- Fasting: 8–12 hours required
- Turn-around: 1-2 business days. May take longer based on weather, holiday or lab delays.
Benefits
- Comprehensive glucose assessment - Four blood draws provide detailed insulin response analysis
- Direct access without physician referral - Skip appointments and get tested immediately
- Up to 80% cost savings versus traditional clinic testing
- Gold standard for gestational diabetes diagnosis - Follows ADA and ACOG guidelines
- Results in 1-2 business days with secure online portal access
- 3,700+ lab locations nationwide for maximum convenience
Who Is This Test For?
- Pregnant women needing gestational diabetes confirmation after abnormal screening
- Adults with diabetes risk factors like obesity, family history, or hypertension
- Individuals with concerning symptoms including frequent urination, excessive thirst, or fatigue
- People with abnormal fasting glucose requiring comprehensive metabolic evaluation
- Those monitoring insulin resistance or prediabetes progression
How It Works – Just 3 Steps
- Fast 8-12 hours and visit any partnered lab for baseline blood draw
- Drink glucose solution (75-100g) and remain at lab for additional draws
- Get results online in 1-2 business days with detailed glucose tolerance analysis
FAQ
What makes this test different from a 1-hour glucose test? This extended version takes four blood samples over three hours, providing more comprehensive glucose metabolism analysis than standard screening tests.
Do I need to stay at the lab during the entire test? Yes, you must remain at the testing facility for all four blood draws taken at precise intervals over three hours.
Can I eat or drink anything during the test? You can only drink water after consuming the glucose solution. No food, gum, or other beverages are allowed.
What if I feel sick during the test? Mild nausea is normal after drinking the glucose solution. Inform lab staff if you vomit, as this may require rescheduling.
How accurate is this test for diagnosing diabetes? The 3-hour GTT has 92% sensitivity and 91% specificity for detecting diabetes, making it highly reliable (Diabetes Care 2021).
More Details
What is the purpose of this test?
The Glucose Tolerance Test (GTT) – 3 Hour evaluates how efficiently your body processes glucose over an extended period. This diagnostic tool measures insulin response and glucose metabolism through four precisely-timed blood samples: one fasting baseline, followed by three additional draws at hourly intervals after consuming a standardized glucose solution.
The test is essential for diagnosing gestational diabetes in pregnant women, confirming prediabetes, and identifying type 2 diabetes in adults with risk factors (ADA Standards of Care 2024). Unlike shorter screening tests, the 3-hour version provides comprehensive glucose metabolism analysis critical for accurate diagnosis.
Who would benefit from this test?
This comprehensive glucose assessment benefits several key groups:
Pregnant women who received abnormal results (≥140 mg/dL) on 1-hour glucose screening tests require this confirmatory test for gestational diabetes diagnosis (ACOG 2023).
Adults with diabetes risk factors including obesity, family history, hypertension, or age over 45 should consider testing. With 98 million U.S. adults having prediabetes—81% unaware of their condition—early detection prevents progression to type 2 diabetes (CDC 2023).
Individuals experiencing diabetes symptoms like frequent urination, excessive thirst, unexplained weight loss, or persistent fatigue need comprehensive glucose evaluation to identify underlying metabolic dysfunction.
When should I order this test?
Consider ordering the 3-hour GTT in these specific situations:
After abnormal glucose screening: When 1-hour glucose tests show elevated results (≥140 mg/dL), this confirmatory test determines if gestational diabetes is present (Mayo Clinic 2024).
With multiple risk factors: Adults with obesity, family diabetes history, hypertension, or metabolic syndrome benefit from comprehensive glucose assessment even without symptoms.
For symptom evaluation: Persistent fatigue, frequent infections, slow wound healing, or unexplained weight changes may indicate glucose intolerance requiring detailed analysis.
Following borderline fasting results: When fasting glucose levels fall between 100-125 mg/dL, the GTT clarifies whether prediabetes or diabetes is present.
How do I interpret the results?
Your results are compared against established diagnostic thresholds to determine glucose tolerance status:
Fasting
- Normal: <95 mg/dL
- Gestational Diabetes: ≥95 mg/dL
- Diabetes: ≥126 mg/dL
1-hour
- Normal: <180 mg/dL
- Gestational Diabetes: ≥180 mg/dL
- Diabetes: ≥200 mg/dL
2-hour
- Normal: <155 mg/dL
- Gestational Diabetes: ≥155 mg/dL
- Diabetes: ≥200 mg/dL
3-hour
- Normal: <140 mg/dL
- Gestational Diabetes: ≥140 mg/dL
- Diabetes: ≥200 mg/dL
Gestational diabetes is diagnosed when two or more values exceed the thresholds above.
For non-pregnant adults, diabetes is confirmed if any single value meets the diabetes criteria. Prediabetes (impaired glucose tolerance) is diagnosed when glucose levels fall between normal and diabetic ranges.
Disclaimer: Reference ranges may vary by laboratory. Listed ranges are general guidelines and may differ from those used by the performing lab. Always consult your healthcare provider for interpretation.
Pre-test preparation
Proper preparation ensures accurate results and successful testing:
Fasting requirements: Fast for 8-12 hours before your appointment. You may drink water but avoid all food, beverages (except water), gum, and mints (University of Michigan 2024).
Dietary preparation: Consume at least 150 grams of carbohydrates daily for three days before testing. This ensures your body responds normally to the glucose challenge.
Medication considerations: Consult your healthcare provider about medications that may affect glucose levels, including steroids, diuretics, and beta-blockers.
Activity restrictions: Avoid vigorous exercise for 24 hours before testing and remain sedentary during the 3-hour testing period. Smoking is prohibited throughout the procedure.
How often should I get tested?
Testing frequency depends on your risk factors and current health status:
- Pregnancy (high risk): 24–28 weeks, repeat if needed
- Prediabetes diagnosis: Every 1–2 years
- Normal results with risk factors: Every 3 years
- Family history of diabetes: Every 2–3 years after age 35
- Gestational diabetes history: Every pregnancy + annual screening
Why early detection matters
Early diabetes detection provides significant health and economic benefits. The annual cost of diabetes in the U.S. reaches $412.9 billion, including direct medical expenses and reduced productivity (ADA 2023).
Preventing complications: Early intervention through lifestyle modifications can prevent or delay type 2 diabetes by up to 58% in high-risk individuals. This reduces risks of heart disease, stroke, kidney disease, and vision problems.
Treatment optimization: Timely diagnosis allows for immediate dietary modifications, exercise programs, and medication when necessary, preventing disease progression and reducing long-term healthcare costs.
Maternal-fetal health: Identifying gestational diabetes early enables proper management to reduce risks of large birth weight babies, cesarean delivery, and future diabetes risk for both mother and child.
Related tests you may consider
Hemoglobin (Hb) A1c Blood Test - Calculates the average blood glucose levels over 2-3 months, making it an ideal diagnostic tool for diabetes management.
Cardio IQ® Insulin Resistance Panel with Score Blood Test - Gives you information to change or avoid becoming diabetic or prediabetic.
Comprehensive Metabolic Panel (CMP-14) with eGFR Blood Test - Evaluates nutrient levels, liver, and kidney function.
