Oral Glucose Tolerance Test
Oral Glucose Tolerance Test
Patient Information
Clinical Biochemistry Department - Pathology at Wigan and Salford
- Author ID: JB
- Leaflet ref: CS 008
- Version: 8
- Leaflet title: Oral Glucose Tolerance Test
- Last review: March 2026
- Expiry Date: March 2028
Why is an oral glucose tolerance test carried out?
Some people’s bodies are unable to use glucose (sugar) properly. This may occur in conditions such as Diabetes Mellitus or Impaired Glucose Tolerance (a condition that can sometimes progress to diabetes). It is important to detect these problems early so that appropriate advice and treatment can be given. Often, a single blood glucose test is enough to identify a problem. However, in some cases, an oral glucose tolerance test is needed to check this more accurately.
What do we look for in this test?
We measure the amount of glucose in your blood before and after giving you a glucose drink. This shows how well your body processes glucose.
Limitations and risks of this test
The accuracy of results depends on the quality of your sample, so it is very important that you follow the instructions carefully both before and during the test.
Some people may experience some discomfort, nausea or light-headedness after drinking the glucose solution. This usually passes quickly.
There is a small chance that you may vomit after the glucose drink. If this happens, please tell the person carrying out the test, as the test will be stopped. If you feel unwell at any time during the test, please tell the person doing the test straight away.
Alternatives to this test
There are no alternatives to this test that provide the same information as an oral glucose tolerance test.
What happens to your results?
Your samples will be tested later in the day, and the results will be sent to your doctor, who will explain what they mean and whether any treatment or follow-up is required. Please make an appointment to see your doctor one week after the test. Laboratory staff will not be able to give you your results, so please wait to discuss them with the doctor who requested for the test to be done.
Your results may be entirely normal.
If the test shows that your body does not clear glucose quickly enough, this may indicate Diabetes Mellitus.
Some people have results that are partly outside of the normal range. This is known as Impaired Glucose Tolerance, which can sometimes progress to Diabetes Mellitus.
Whatever your results, your doctor will discuss any lifestyle changes that may help, such as diet, monitoring and possibly other treatments.
Instructions
Please read the information and follow the instructions fully, as not doing so may lead to an inaccurate result and the need to repeat the test.
Before coming for the test
- For the 3 days before the test, eat your normal diet
- Take only your usual amount of exercise during these 3 days
- Do not eat anything from 10:00 pm the night before the test until after the test is completed.
- If you regularly take medicines, continue taking them as usual.
- Please do not smoke until the test is finished.
- You may drink plain, unflavoured water freely, but no other drinks
- If any of your medicines need to be taken with food, you may need to wait until after the test to take them.
- If you are unsure, please contact your doctor.
- Please bring a list of any medicines that you take and give the list to the person carrying out the test, as some medicines may affect the test.
The test itself
When you arrive at the clinic, you will have a blood sample taken. This will be tested immediately. What happens next depends on the result of this first test. In most cases, you will then be asked to drink a glucose solution, after which two further blood samples will be taken – one after 1 hour and another 2 hours after the drink.
There is a lot of waiting between these blood samples, but it is important for the accuracy of the test that you sit quietly during this time. You may wish to bring something to read.
You must continue to fast (nothing to eat or drink except plain water) until the test is fully complete. You may wish to bring a bottle of water with you. Smoking is not permitted during the test.
How long will it take?
The test usually takes around 2 ½ hours to complete. In some cases, it may not be necessary to perform the full test, depending on the results of the first blood sample. If this happens, you may be given an appointment for another fasting blood test instead.
If you require further information about this test, please contact the:
Clinical Biochemistry Department
Pathology at Wigan and Salford
Salford Royal NHS Foundation Trust
Stott Lane
Salford
M6 8HD
Telephone: 0161 206 4958 or 0161 206 4959