What is an oral glucose tolerance test (OGTT)?

An oral glucose tolerance test, or OGTT, is a test that looks at the amount of glucose (sugar) in your blood. The results of the test can be used to make a diagnosis of diabetes, including gestational diabetes, which is diabetes that develops during pregnancy.

You may be referred by your GP to take this test if you have risk factors for developing diabetes or you have had previous elevated blood sugar levels.

All pregnant women are also recommended to complete an OGTT during their pregnancy. This usually happens around 24 – 28 weeks.

Some women who are more at risk of having gestational diabetes need to do an OGTT early in their pregnancy.

You are more at risk if:

  • Your Body Mass Index (BMI) is 30 or above
  • You’re aged over 35
  • Someone in your immediate family has diabetes
  • You have Aboriginal or Torres Strait Islander, Māori, Pasifika, South Asian, South-east Asian, Latin American or Middle Eastern heritage
  • You’ve had gestational diabetes in a previous pregnancy
  • You’ve given birth to a baby weighing 4.5kg or more
  • You take certain medications (antipsychotic medications, steroid medications)
  • You have had previous cardiovascular disease (heart attack, stroke)

You can do the test at your medical centre, pathology or local hospital.

What do I need to do before my oral glucose tolerance test?

You will usually need to fast the night before the test. Fasting means not eating or drinking anything except water. You can take your usual medicine.

The test takes two-to-three hours, so it’s a good idea to think about what you can take to keep you busy (magazine, book, tablet). You can’t breastfeed during the test, so if you are breastfeeding, make sure you feed before the test begins. You can bring some stored breast milk in case your baby gets hungry.

What will happen during my oral glucose tolerance test?

  • A nurse will take a blood sample when you arrive – this measures the ‘resting’ level of sugar in your blood
  • You’ll be given a sweet drink, containing 75g glucose, which tastes like flat lemonade. You will need to drink all of it
  • A nurse will test your blood one and two hours after you drink the sugary drink
  • The whole test may take 2.5 to 3 hours
  • You might feel unwell after the sugary drink – this is normal
  • If you are pregnant, your baby may move around more during the test – this is normal
  • You shouldn’t eat or drink until the test is finished

What can I expect after my oral glucose tolerance test?

You will receive the test results within 48 hours. The results show how your body handled the glucose to reveal if you have diabetes. If you test negative, there will be no change to your pregnancy care.

The test can also identify if you have prediabetes. Your GP will advise you on how you can manage your blood sugar levels.
If you are pregnant, the test can be used to identify if you have gestational diabetes. If you test negative, there will be no change to your pregnancy care.
If you test positive, your GP, midwife or obstetrician will talk to you about what that means. You’ll be given help to manage your condition. Your care team may refer you to a specialist clinic to help manage the gestational diabetes.