Glucose tolerance tests: What exactly do they involve?

Oral glucose tolerance tests (OGTT) are used to measure how well the body can process a larger amount of sugar. If the blood sugar measured in the test is above a certain level, this could be a sign that sugar is not being absorbed enough by the body’s cells. Diabetes or gestational diabetes might be at the root of this problem.

In gestational diabetes, blood sugar levels are often higher due to changes in the metabolism during pregnancy – but they usually come back down again after the child is born.

What types of glucose tolerance tests are there?

There are two types of glucose tolerance tests: a short version called the glucose challenge test, and a full glucose tolerance test. The short version is easier to do and serves as a preliminal test to determine someone's risk of diabetes or gestational diabetes.

Glucose challenge test

The glucose challenge test is the short version of the glucose tolerance test. The test can be done at any time of the day. It involves drinking a glass of concentrated glucose solution (50 g of glucose dissolved in 250 to 300 ml of water). After one hour has passed, a blood sample is taken to determine the blood sugar level.

Glucose tolerance test

For this test, you should not eat anything before going to the doctor in the morning. In other words, you should not have breakfast, and you should eat your last meal the evening before. This also applies to all drinks with the exception of water.

First of all, blood is taken to determine your baseline blood sugar level. The blood is drawn from a vein or your fingertip or earlobe. After that you drink a large glass of concentrated sugar solution. In the glucose tolerance test, 75 g of glucose are dissolved in 250 to 300 ml of water. The amount given to children is based on their body weight. If the test is being done to confirm suspected diabetes, blood is drawn again after two hours and the blood sugar level is measured. When testing for gestational diabetes, blood is drawn twice – first after one hour and then again after another two hours.

It is recommended that you do the test while lying down or sitting, and do not eat, drink or smoke until the last blood sample is taken. It is also important that you maintain a normal, balanced diet in the days leading up to the test. Major changes in your eating habits, like going on a diet, can influence the results of the test and make them less reliable. Some medications can also distort the results. You can discuss what exactly you need to be careful about ahead of time with your doctor.