Oral Glucose Tolerance Test (GTT)
You have been asked to come to the hospital for an oral Glucose Tolerance Test (GTT). This page gives you information about the test and the preparations needed.
What is the oral GTT?
This test is a way of looking at how your body handles glucose (sugar). You will be given a glucose drink called Polycal®. Blood samples will be taken before having the drink and 2 hours afterwards. The levels of glucose in the blood samples will be measured in the laboratory. Your healthcare professional will check the results to see how well your body is able to manage your blood sugar levels.
Why am I having a GTT?
There are a number of reasons why your doctor thinks you need to have a glucose tolerance test:
- If you have symptoms or have had other tests that suggest that you may have diabetes mellitus (sometimes just called diabetes). GTT is a useful way of making the diagnosis.
- Some people develop a condition called gestational diabetes during pregnancy. It is important for the wellbeing of both you and the baby that this is found quickly so that treatment can be given.
- The GTT may sometimes be used to test other hormone systems, such as looking at growth hormone release. This test is not common but will be discussed with you if needed.
The healthcare professional should have discussed the risks and benefits of the GTT with you. If you have any questions about the GTT, the risks and benefits or why you are having this test, please contact them.
What you must do before coming for the GTT
- You should eat normally for at least 3 days before the test and should not have been dieting.
- The test will normally be scheduled in the morning. For oral GTT tests scheduled at any other time of the day, please discuss with your healthcare professional what you must do before coming for the test.
- If the GTT is to take place in the morning, then on the day before the test is due:
- Stop eating food by 10 o’clock at night.
- You can drink water but must avoid all other drinks.
- Do not chew gum or eat sugar free sweets.
- Do not smoke or use e-cigarettes or nicotine patches.
- Other than water, do not have anything to eat or drink on the morning of the test.
- Check with your healthcare professional that you can delay taking any tablets or medicines on the day of the GTT test until after the test has been completed.
What will happen during the test?
A blood sample will be taken from a vein in your arm. You will then be asked to drink a glass of diluted Polycal® followed by a small drink of water.
The diluted Polycal® is a flavoured drink containing the equivalent of 75g of glucose.
Two hours after finishing the drink, a second blood sample will be taken.
Further blood samples may be taken if you are having one of the special tests, such as for looking at your growth hormone release, but this will have been discussed with you beforehand.
All blood samples taken will be sent to the laboratory for glucose analysis. You must sit quietly and must not smoke, chew gum or eat sweets during the 2 hours of the test.
If you feel unwell at any point during the test, please let a healthcare professional know straightaway.
When will I have the results of the tests?
The laboratory staff cannot give you the results of the test. The results will be sent to your healthcare professional, normally within 24 hours. Your healthcare professional will then discuss the results with you.
If you need to cancel a GTT appointment
If you cannot attend the test for any reason, please contact the doctor who advised you to have the test.
Important points to remember
- If the test is scheduled for the morning, have nothing to eat or drink other than water from 10 o’clock the night before the test until after the test is finished. For oral GTT tests scheduled at other times, discuss requirements with your healthcare professional.
- You may want to bring something with you to read during the test as you will need to rest while waiting to have the second blood sample taken.
- You will be in the department for up to 3 hours, so please bear this in mind when parking if you have travelled by car.
Further information
If you would like to learn more about this test, please visit the following website:
Lab Tests Online UK
This is a free resource created and updated by laboratory professionals for patients.