Oral Glucose Tolerance Test
Also written as: OGTT, GTT, Glucose Tolerance Test
Timed glucose measurements after a standard sugary drink, showing how the body handles a sugar load.
What it measures
Blood glucose measured before, and at set times after, drinking a standard measured glucose drink. Rather than a single snapshot, it records how quickly the body moves a known sugar load out of the blood.
Why a doctor orders it
Most commonly ordered during pregnancy to screen for gestational diabetes. Also used when fasting results are borderline or do not match the overall picture, since some glucose handling problems appear only after a sugar load.
What is generally associated with a higher result
Higher post-drink values are broadly associated with a reduced ability to clear glucose from the blood. Steroid medicines, acute illness, prolonged inactivity, and a low carbohydrate diet in the days beforehand are all linked with higher readings.
What is generally associated with a lower result
A brisk fall after the drink is broadly associated with vigorous insulin release. The shape of the whole curve, together with how the person felt during the test, generally matters more than any single point.
Everyday things that move this number
Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:
- Carbohydrate restriction in the days before, which can exaggerate the response
- Not fasting properly beforehand
- Illness, stress, or poor sleep around the test
- Walking around, smoking, or eating during the test
- Vomiting the drink, which invalidates the result
Questions to take to your appointment
- Should I eat my normal diet, including carbohydrates, in the days before the test?
- How long is the fast beforehand, and is water allowed?
- What happens if I feel unwell or vomit during the test?
- Am I allowed to move around between the blood draws?
Build a printable questionnaire with your own results →
Where to read more
MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .
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