This article provides a guide to the glucose tolerance test, a test for identifying diabetes, pre-diabetes and gestational diabetes. We explain what the test involves, the types of tests available, how to prepare for the tests and what the results mean. We also list the symptoms of hyperglycaemia (high blood glucose levels) and hypoglycaemia (low blood glucose levels) and what they might mean for your health. Lastly, we offer guidance on what to do if your test results indicate that you are at risk of developing type 2 diabetes. We have also included information about the Life! program, a free healthy lifestyle program that helps to prevent diabetes, heart disease and stroke.

What is a glucose tolerance test?

A glucose tolerance test (or GTT) checks the level of glucose (aka sugar) in your blood. 

GTTs are vital in diagnosing: 

  • type 1 diabetes
  • type 2 diabetes 
  • pre-diabetes
  • gestational diabetes. 

Why have a glucose tolerance test?

Glucose tolerance tests show whether or not your blood glucose level is normal. This is extremely important.

Glucose is the body’s main source of energy. When we eat or drink, glucose enters our bloodstream, causing the pancreas to release the hormone insulin. Insulin controls the amount of glucose in the blood. This is vital as most of the cells in our body can only function when our glucose levels stay within a certain range. 

If the glucose/insulin system is working properly, the amount of glucose in the blood is correctly controlled. When it isn’t working properly and a person has a condition called insulin resistance, the amount of glucose in the blood can rise or fall.

Having high blood glucose levels is called hyperglycaemia; having low blood glucose is called hypoglycaemia. Both conditions can cause serious damage to the body.  

When to get tested

Your doctor will advise you to have a GTT if you have symptoms of hypoglycaemia or hyperglycaemia, or if you are pregnant. If you already live with diabetes, you may need regular GTTs.

The most common early signs of hypoglycaemia (low blood glucose) are:

  • sweating
  • feeling shaky and unsteady
  • having a fast heart beat
  • suddenly feeling hungry
  • headache.

Hypoglycaemia can progress to the point where the brain is not getting enough glucose to function properly. This is known as severe hypoglycaemia, and its symptoms include:

  • being confused/unable to think clearly
  • slurred speech
  • having a seizure
  • becoming unconscious.

The most common symptoms of hyperglycaemia (high blood glucose) are:

  • frequently needing to urinate
  • increased thirst
  • tiredness
  • weight loss
  • blurred vision
  • infections

Untreated hyperglycaemia can cause harmful acids to build up in your blood and urine. Symptoms include:

  • feeling very thirsty
  • having a dry mouth
  • nausea and vomiting
  • abdominal pain
  • shortness of breath
  • weakness
  • confusion
  • becoming unconscious.

Types of glucose tolerance test

There are three main types of GTTs.

  1. Fasting glucose test (FGT)/blood glucose test.
  2. Oral glucose tolerance test (OGTT). 
  3. HbA1C test. 

All three types are simple and produce fast results. These tests will give a good indication of whether you have pre-diabetes or one of the types of diabetes.

  • Fasting glucose test (FGT)/blood glucose test

A blood sample is taken to show the level of glucose in your blood after fasting (not eating or drinking anything other than water) for 8-10 hours. 

A fasting glucose test measures how much glucose is in your blood at the moment the test is carried out. 

  • Oral glucose tolerance test (OGTT)

The level of glucose in your blood is checked after fasting for 8-10 hours. You then have a glucose drink (usually with 75 grams of glucose) and a further blood sample is taken after 1 or 2 hours. 

This test shows how your body responds to glucose after a meal.

An OGTT is sometimes used to diagnose pre-diabetes and diabetes. The test is also routinely offered to pregnant women to check for gestational diabetes.

  • HbA1c test 

The HbA1c test checks your average blood glucose level over the past three months. It is a simple blood test that can be arranged by GP, and no fasting is required. For people who live with diabetes, this shows how well the condition is being controlled. It is also used to diagnose and monitor pre-diabetes and diabetes.

What do the results mean?

Your doctor will look at your blood test results and explain what they mean for you.

If your blood glucose level is high, you may have diabetes, pre-diabetes or gestational diabetes. But other diseases and conditions can also cause raised blood glucose, so your doctor may suggest more testing to exclude them.

If you do have pre-diabetes, your doctor will recommend making healthy lifestyle changes, such as:

  • following a healthy eating pattern
  • increasing your physical activity
  • reaching and maintaining a healthy weight
  • stopping smoking.

These changes may help your body use insulin more effectively and prevent pre-diabetes from progressing to type 2 diabetes.

What should I do if my glucose tolerance test results show I am at risk of type 2 diabetes? 

If your GTT results show that you are at risk of developing type 2 diabetes, you can take positive action by finding a healthy lifestyle program.

The Life! program is a free lifestyle improvement program. It will help you to prevent diabetes, heart disease and stroke.

Our experienced health professionals will support you to make changes to your lifestyle that will reduce your risk of type 2 diabetes, heart disease and stroke.

The Life! program is the largest prevention program of its kind in Australia. Since 2007, over 75,000 Victorians have joined the program to learn more about the steps they can take to live a healthy life. 

You can take a quick online health test and check your eligibility for the program here

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Angel Fan, Accredited Practicing Dietitian | Primary Care Engagement Lead

Kristie Cocotis, Head of Prevention and Health Promotion

Sarah Dubé, Strategy and Engagement Lead

Ria Cheripuram, Digital Communications Officer

Tegan Kohlman, Communications and Social Marketing Officer