Oral glucose tolerance test (OGTT)
Challenge test that shows how the body handles a set amount of sugar
Also known as
Definition
In the oral glucose tolerance test, you drink a sugar solution, usually containing 75 g of glucose, after fasting for at least 8 hours. Blood sugar is measured before and after. This shows how well your body moves sugar from the blood into tissues such as muscle and fat. It is mainly used to check for diabetes. There is a separate procedure for pregnant people that is not described here.
| Parameter | Value |
|---|---|
| Unit | mmol/l (mg/dl) |
| Reference Range | Typical values for a 75 g OGTT in adults who are not pregnant: fasting 3.3–5.5 mmol/l (60–99 mg/dl) | after 1 hour below 11.1 mmol/l (200 mg/dl) | after 2 hours below 7.8 mmol/l (140 mg/dl). Labs may differ slightly; the range on your own report is what counts. Your doctor interprets the result. |
Agree on your personal target range with your doctor.
Conversion for glucose: 1 mmol/l = 18.02 mg/dl.
↓ What a low value means
The test is mainly used to detect values that are too high. Values in the normal range suggest that diabetes is probably not present. A value below your lab's range may be related to medicines that lower blood sugar, among other things; vigorous exercise can also lower blood sugar over several hours. If blood sugar drops very low, it can lead to loss of consciousness or a seizure. Discuss any deviations with your doctor.
↑ What a high value means
Higher values may point to prediabetes or diabetes. A 2-hour value of 140–199 mg/dl (7.8 to below 11.1 mmol/l) is called impaired glucose tolerance, often also referred to as prediabetes; the risk of developing diabetes over time is then increased. A glucose value of 11.1 mmol/l (200 mg/dl) or higher is considered diagnostic of diabetes. Serious physical stress such as injury, stroke, heart attack or surgery, vigorous exercise right before the test and some medicines can raise blood sugar. Your doctor makes the diagnosis.
When to test
The OGTT is mainly considered when fasting blood sugar is raised but below the diabetes threshold of 7.0 mmol/l (126 mg/dl). A modified version is used when growth hormone excess (acromegaly) is suspected. To prepare, eat as usual in the days before and fast for at least 8 hours; you do not eat during the test. Ask beforehand whether your medicines can affect the result.
Frequently asked questions
How does an OGTT work? +
First, a fasting blood sample is taken. Then you drink a sugar solution, usually with 75 g of glucose, which tastes similar to a very sweet soda. Further blood samples follow, depending on the procedure after two hours or every 30 to 60 minutes. The test can take up to three hours.
Why an OGTT when there are fasting glucose and HbA1c? +
Fasting glucose and HbA1c are usually the first tests. The OGTT also shows how the body copes with a sugar load. Impaired glucose tolerance is considered an earlier sign of diabetes than an abnormal fasting value. That is why the test is often used when the fasting value is raised but not yet in the diabetes range.
Does the test have side effects? +
Serious side effects are very uncommon. After drinking the solution, some people feel nauseated, sweaty or lightheaded; in rare cases, shortness of breath or fainting occurs. Let the staff know beforehand if you have had such reactions to blood draws or medical procedures.
What can affect the result? +
Serious physical stress such as injury, stroke, heart attack or surgery can raise blood sugar. Vigorous exercise can lower it over several hours but may also raise it immediately afterwards. Some medicines can raise or lower blood sugar; only stop them after talking to your doctor.
Oral glucose tolerance test (OGTT)
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.