What do glucose tolerance tests involve?

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, serving as a preliminary test to determine someone's risk of diabetes or gestational diabetes.

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 (sugar) solution made from 50 grams (g) of glucose dissolved in 250 to 300 milliliters (ml) of water. After one hour has passed, a blood sample is taken to determine the blood sugar level.

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 is also true for all drinks apart from 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 sugar solution. In the glucose tolerance test, 75g of glucose are dissolved in 250 to 300ml 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 one hour after drinking the solution 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 eat a normal 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 to look out for ahead of time with your doctor.