What do glucose tolerance tests involve?

Oral glucose tolerance tests (OGTT) are done to measure how well the body can process larger amounts of sugar. If the blood sugar reading is above a certain level, this could be a sign that sugar is not being absorbed fast enough by the body’s organs. Then diabetes or gestational (pregnancy-related) 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, serving as a preliminary 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 (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.

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 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.

Bundesärztekammer (BÄK), Kassenärztliche Bundesvereinigung (KBV), Arbeitsgemeinschaft der Wissenschaftlichen Medizinischen Fachgesellschaften (AWMF). Nationale Versorgungsleitlinie: Therapie des Typ-2-Diabetes. S3-Leitlinie. AWMF-Registernr.: nvl-001g. 2023.

Deutsche Diabetes Gesellschaft (DDG), Deutsche Gesellschaft für Gynäkologie und Geburtshilfe (DGGG). Gestationsdiabetes mellitus (GDM): Diagnostik, Therapie und Nachsorge (S3-Leitlinie). AWMF-Registernr.: 057-008. 2018.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

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Updated on December 18, 2023

Next planned update: 2026


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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