Glucose Tolerance Test: Types, How to Prepare, What Happens and What Results Mean

7 days ago

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Updated: 2 days ago

Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: November 2026 · How we review

Quick Answer

A glucose tolerance test shows how well the body clears sugar from the blood. For the standard 2-hour test, you fast overnight, drink 75 grams of glucose and have blood drawn after 2 hours. A 2-hour value under 140 mg/dL is normal, 140–199 mg/dL means prediabetes, and 200 mg/dL or higher means diabetes.

What the research says

The 1-hour value is an early warning: an International Diabetes Federation position statement concludes that a 1-hour plasma glucose of 155 mg/dL (8.6 mmol/L) or more after a 75 g load identifies intermediate hyperglycemia and predicts type 2 diabetes and complications earlier than the 2-hour value, and that 209 mg/dL (11.6 mmol/L) or more is diagnostic of type 2 diabetes (Bergman M et al., Diabetes Research and Clinical Practice, 2024) [2]
Prediabetes is not a one-way street: a Cochrane review of 103 prospective cohorts found that people with both impaired fasting glucose and impaired glucose tolerance progress to diabetes most often, but many people with prediabetes return to normal glucose levels, especially within the first years (Richter B et al., Cochrane Database of Systematic Reviews, 2018) [3]

Key takeaways

Eat normally (at least 150 g of carbohydrate a day) for 3 days before, then fast 8–12 hours.
The 75 g drink must be finished within 5 minutes; stay seated and do not eat, smoke or exercise during the test.
2-hour results: under 140 normal, 140–199 prediabetes, 200 or more diabetes (mg/dL).
In pregnancy, a 1-hour 50 g screen (no fasting) may be followed by a 3-hour 100 g test.
Any abnormal result is confirmed by repeating the test on another day.

What is a glucose tolerance test?

Fasting glucose and A1C show your baseline and your 3-month average. The OGTT adds a stress test: it measures how your insulin response handles a measured sugar load. It is more sensitive than fasting glucose for detecting impaired glucose tolerance and is the standard test for gestational diabetes. Sources: [1]

Four-step overview of the oral glucose tolerance test: preparation, baseline draw, glucose drink, timed draws

The 2-hour OGTT is usually done in the morning after an overnight fast.

Types of glucose tolerance tests

List of glucose tolerance test types including the 2-hour 75 g OGTT, the 1-hour pregnancy screen and the 3-hour test

The protocol depends on what your provider is checking.

Test	Who it is for	Protocol
2-hour 75 g OGTT	Adults with suspected prediabetes or type 2 diabetes; also used in pregnancy in a one-step approach	Fasting draw, 75 g drink, draw at 2 hours (sometimes 1 hour)
1-hour 50 g screen	Pregnancy, 24–28 weeks	No fasting; draw 1 hour after the drink
3-hour 100 g OGTT	Pregnancy, after an abnormal 1-hour screen	Fasting draw, 100 g drink, draws at 1, 2 and 3 hours
Growth hormone suppression test	Suspected acromegaly	Growth hormone measured at 0, 30, 60, 90 and 120 minutes after glucose

Sources: [1]

How do I prepare?

For 3 days before, eat your usual diet with at least 150 g of carbohydrates per day; a low-carb diet can give a falsely abnormal result.
Fast for 8–12 hours before the test (water only); for the 1-hour pregnancy screen, no fasting is needed.
Ask your provider about medicines that affect glucose, such as steroids, diuretics and some antidepressants.
Do not smoke or exercise vigorously the morning of the test.
Bring something to read; you will be at the lab for 2–3 hours. Sources: [1]

What happens during the test?

You arrive in the morning, usually between 7 and 9 a.m., and a fasting blood sample is taken.
You drink a sweet solution containing 75 g of glucose in 250–300 mL of liquid within 5 minutes.
You sit quietly; walking around, eating or smoking can change the result.
Blood is drawn again at 2 hours (and at 1 hour, or at 1, 2 and 3 hours for the 100 g test).
You can eat and resume normal activity afterward.

Some people feel nauseated, sweaty or light-headed from the drink. Tell the staff if you vomit, because the test will need to be rescheduled. Sources: [1]

Understanding your results

Three stat cards showing 2-hour OGTT thresholds for normal, prediabetes and diabetes

Thresholds for the 2-hour plasma glucose in non-pregnant adults.

2-hour plasma glucose	mg/dL	mmol/L	Interpretation
Normal	below 140	below 7.8	Normal glucose tolerance
Prediabetes	140–199	7.8–11.0	Impaired glucose tolerance
Diabetes	200 or higher	11.1 or higher	Diabetes (confirm on another day)

Thresholds in pregnancy are lower and depend on which protocol your provider uses; see our guide to the glucose test in pregnancy. Sources: [1]

Newer evidence highlights the 1-hour value: a 1-hour glucose of 155 mg/dL or more predicts future diabetes and complications even when the 2-hour value is normal, and 209 mg/dL or more is considered diagnostic. On the other hand, a prediabetes result is not a sentence; across 103 cohorts, many people returned to normal glucose levels, particularly in the first few years. Sources: [2, 3]

Risks

The test is safe. The main downsides are the time it takes and temporary nausea, bloating or light-headedness from the sugary drink. The blood draws carry only minor bruising risk. Sources: [1]

Conclusion

The glucose tolerance test remains the most sensitive routine way to find early problems handling sugar, which is why it is used in pregnancy and whenever fasting glucose and A1C leave doubt. Prepare properly, sit still during the test, and remember that any abnormal result is confirmed with a repeat before a diagnosis is made.

Keep learning: read about prediabetes and gestational diabetes.

Frequently asked questions

How long does a glucose tolerance test take?

About 2 hours for the standard test, 1 hour for the pregnancy screen and 3 hours for the 100 g confirmatory test, plus time for the drink and blood draws.

Do I have to fast?

Yes for the 2-hour and 3-hour tests (8–12 hours, water only). The 1-hour 50 g pregnancy screen does not require fasting.

What is a normal 2-hour glucose result?

Under 140 mg/dL (7.8 mmol/L). Values of 140–199 mg/dL indicate prediabetes and 200 mg/dL or more indicate diabetes.

Can I eat a low-carb diet before the test?

No. Eating fewer than 150 g of carbohydrates a day for several days can cause a falsely high result. Eat normally for 3 days before.

What does a 1-hour value of 155 mg/dL mean?

According to the International Diabetes Federation, it signals intermediate hyperglycemia and a higher risk of future diabetes, even if the 2-hour value is normal.

What if I vomit the drink?

The test cannot be interpreted and will need to be repeated on another day.

Is the test used in pregnancy?

Yes. A 1-hour 50 g screen at 24–28 weeks may be followed by a 3-hour 100 g test, or a single 2-hour 75 g test is used, depending on your provider.

Can prediabetes go away?

Yes. Cochrane evidence shows many people with prediabetes return to normal glucose, especially in the first few years. Weight loss and activity can also help.

Why not just check A1C?

A1C reflects average glucose but can miss impaired glucose tolerance and is unreliable in some conditions. The OGTT directly tests how your body handles sugar.

What is the growth hormone suppression test?

A version of the OGTT used to diagnose acromegaly. Growth hormone normally falls after glucose; if it does not, excess growth hormone is likely.

References

Cleveland Clinic. Glucose Tolerance Test (GTT): What It Is, Preparation & Results. Cleveland Clinic. Last updated August 16, 2024. https://my.clevelandclinic.org/health/diagnostics/glucose-tolerance-test
Bergman M, Manco M, Satman I, et al. International Diabetes Federation position statement on the 1-hour post-load plasma glucose for the diagnosis of intermediate hyperglycaemia and type 2 diabetes. Diabetes Research and Clinical Practice. 2024;209:111589. PMID 38458916. https://doi.org/10.1016/j.diabres.2024.111589
Richter B, Hemmingsen B, Metzendorf MI, et al. Development of type 2 diabetes mellitus in people with intermediate hyperglycaemia. Cochrane Database of Systematic Reviews. 2018;10(10):CD012661. PMID 30371961. https://doi.org/10.1002/14651858.CD012661.pub2

Medical disclaimer

This article is for general education only and does not replace advice from a qualified healthcare professional. Talk to your doctor about your own diagnosis and treatment, and seek emergency care right away for severe or sudden symptoms.