OGTT (Glucose Tolerance Test)
Also known as: Glucose tolerance test, GTT, 75 g OGTT, OGTT test, pregnancy mein sugar test, glucose pee kar test
The oral glucose tolerance test. Blood sugar is measured fasting, then you drink 75 g of glucose and blood sugar is measured again, usually after 2 hours (and after 1 hour in pregnancy).
Why it's ordered
Diagnosing diabetes and prediabetes when other tests are borderline, and screening for diabetes in pregnancy (gestational diabetes), usually at 24–28 weeks.
Preparation
Eat normally (including carbohydrates) for 3 days before. Fast overnight for 8–10 hours; water is allowed. During the test, sit quietly, don't eat, smoke or walk around. Tell the lab if you already have diabetes; the test may not be needed.
Typical adult values
| Not pregnant, 2-hour | below 140 mg/dL normal; 140–199 prediabetes; 200 or higher diabetes |
|---|---|
| Pregnancy (75 g), any one of | fasting 92 or higher, 1-hour 180 or higher, 2-hour 153 or higher (mg/dL) = gestational diabetes |
Ranges vary between labs and methods. Always use the range printed on your report.
If it's high
A 2-hour value of 200 mg/dL or higher suggests diabetes (usually confirmed with a repeat test). In pregnancy, one value at or above the limits means gestational diabetes, which needs diet advice, monitoring and sometimes medicine to protect mother and baby.
If it's low
A low value at 2 hours is usually not important, but feeling shaky, sweaty or faint during the test should be reported to the staff straight away.
Good to know
Women who had gestational diabetes have a much higher risk of type 2 diabetes later and should be tested again after delivery and then regularly. Vomiting during the test makes the result invalid.
Common questions
How should I prepare for an OGTT?
Eat normally for 3 days before, then fast overnight for 8–10 hours; water is allowed.
What 2-hour OGTT result means diabetes?
200 mg/dL or higher, usually confirmed with a repeat test; 140–199 is prediabetes. Always use the range printed on your report.
What are the OGTT limits in pregnancy?
With 75 g glucose, any one of fasting 92 or higher, 1-hour 180 or higher, or 2-hour 153 or higher (mg/dL) means gestational diabetes. Always use the range printed on your report.
When is gestational diabetes tested?
Usually at 24–28 weeks of pregnancy.