Blood test
Glucose Tolerance Test
A glucose tolerance test checks how your body handles sugar by measuring blood glucose before and after a sugary drink, used mainly to diagnose diabetes and gestational diabetes.
Quick answer
Glucose Tolerance Test: what it is and what the results mean
An oral glucose tolerance test (OGTT) measures your blood sugar while fasting and again a set time (usually two hours) after you drink a standard sugary solution, showing how well your body processes glucose.
- Why it is done: It is used to diagnose diabetes and impaired glucose tolerance, and is a key test for gestational diabetes in pregnancy, particularly when fasting glucose or HbA1c results are borderline.
- Understanding results: Results are compared with diagnostic thresholds for fasting and post-drink glucose.
What it is
An oral glucose tolerance test (OGTT) measures your blood sugar while fasting and again a set time (usually two hours) after you drink a standard sugary solution, showing how well your body processes glucose.
Why it is done
It is used to diagnose diabetes and impaired glucose tolerance, and is a key test for gestational diabetes in pregnancy, particularly when fasting glucose or HbA1c results are borderline.
What to expect
You fast overnight beforehand. A fasting blood sample is taken, you drink the glucose solution, then you wait (usually two hours, resting) before further blood samples. The appointment lasts a couple of hours.
Understanding the results
Results are compared with diagnostic thresholds for fasting and post-drink glucose. Values above the cut-offs indicate diabetes or impaired glucose tolerance; your clinician explains what yours mean and any next steps.
Good to know
Risks and limitations
It is a safe test, though the sugary drink can cause temporary nausea or light-headedness. It requires fasting and time, and results can be affected by illness, so it is done when you are otherwise well.
Education and reference only. This explains the test in general terms and is not medical advice. Always follow the specific instructions from the team arranging your test, and discuss your results with your clinician.
Answers
Glucose Tolerance Test: frequently asked questions
Why do I have to fast before the test?
Fasting gives an accurate baseline blood sugar so the rise after the glucose drink can be measured properly. Eating beforehand would make the results unreliable.
Can I eat or move around during the two-hour wait?
No. You should not eat, and you are usually asked to rest quietly, because food and vigorous activity affect blood sugar and would change the result.
Related tests
Other blood test investigations
Sources
Where this is drawn from
- NHS — Tests and treatments
- NICE — diagnostic guidance
- Association for Laboratory Medicine / relevant professional body
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