What to expect from a glucose tolerance test

I have been told I need a ‘GTT’ – what on earth is that and what is it for?

5 things to know:

1. You must make sure you not eat anything after 10pm the night before the test. 
2. You can drink as much water as you need to, just make sure it doesn’t contain anything else
3. If you forget, ask for the test to be rebooked.
4. Some people find the fasting times difficult to manage. If this is you, make sure you tell the midwife, as there is an alternative.
5. The alternative, if you cannot tolerate the test, is to do some blood glucose monitoring. This is more cumbersome as it means testing your blood sugar with a finger prick before and after each meal, and first thing in the morning and at bed time for a week or so. 

What is a GTT?

GTT or OGTT stands for glucose tolerance test, or oral glucose tolerance test. This is a specialist test used to look for diabetes. It tests the way that your body handles sugar in your diet. This is very important in pregnancy as, if your body cannot handle sugar well and you are developing diabetes, this can cause complications for you and your baby during pregnancy and birth. You will usually be in the hospital for a couple of hours while the test is carried out. A blood test will be taken to start with, which measures the baseline amount of sugar in your blood. You will then be given a sugary drink which has a known amount of sugar in it. Two hours later, another blood test will be taken to measure how well your body has handled the sugary drink. Some hospitals also test one hour after the sugary drink. It is really important that you don’t eat anything else until after the last blood test is taken, otherwise the result will be affected.

When the test is done for pregnancy, it is for women who are at risk of gestational diabetes (GDM). This just means diabetes that has developed during pregnancy. You will only be tested if you are at risk of GDM, so not everyone will have to undergo this.

Who may be offered a GTT?

• Those with a body mass index (BMI) of 30 or more
• Those women who have had a previously big baby who weighed 4.5kg or more
• Those with a family history of diabetes in a first degree relative. This means if your mum, dad, brother or sister have diabetes.
• If your family origin has a higher risk of diabetes which includes women from South Asia, Black Caribbean or middle Eastern
• Those women who have had gestational diabetes in a previous pregnancy

When is it done?

The test is usually done around 26 weeks of pregnancy. However, if your medical team thinks you are at particularly high risk, it may be suggested that you have the test a bit earlier at 16 weeks. This is usually the case if you have had diabetes in a previous pregnancy. This is because most women who have had diabetes in a previous pregnancy are more likely to get it again in future pregnancies. 

How is it done?

The way the test has to be carried out can be quite complicated, and it is quite important to follow the instructions. If you don’t, it can lead to inaccurate results (ie you being diagnosed with diabetes when you don’t have it) or the test needing to be repeated.

You will be asked to come to your appointment having fasted from around 10pm the night before. Make sure that you eat and drink your usual diet, and do the usual amount of exercise in the days before the test. You can drink water but don’t drink anything which has any sugar or energy in it at all. If you do forget to fast and eat or drink, do tell the midwives taking the test. It is much better to reschedule the test for another day than get an inaccurate result.