Glucose Tolerance Test (OGTT)

What is a glucose tolerance test?

A glucose tolerance test, also known as an oral glucose tolerance test (OGTT), is used to detect or confirm diabetes. It is commonly used to screen for type 2 diabetes and gestational diabetes, a type of diabetes that develops in pregnant women.

How it works

The OGTT measures how your body responds to sugar. More specifically, glucose is the type of sugar that your body uses to supply energy to its cells. Your body breaks down the food that you eat into glucose, which then enters your bloodstream. A hormone called insulin helps bring the glucose from your bloodstream into the cells of your body.

If your body has difficulty responding to insulin, or if your body cannot produce enough insulin, the glucose levels in your blood will be too high. This can be a sign of pre-diabetes or diabetes. Diabetes is a serious medical condition that can damage organs and tissues in your body if uncontrolled.

Why do you need a glucose tolerance test?

You might need an OGTT if you:

• Are overweight or obese
• Have high blood pressure
• Have a family history of diabetes
• Have polycystic ovarian syndrome
• Have high triglycerides, a type of fat in your blood
• May have dumping syndrome, where food moves from your stomach into your small bowel too quickly
• Had gestational diabetes during a previous pregnancy

The above are risk factors for diabetes and gestational diabetes. You may order an OGTT to screen and diagnose these 2 conditions.

What are the risks and complications of a glucose tolerance test?

The OGTT causes very few issues for patients who complete this test. Side effects, if any, are usually minor.

After your blood is drawn, you may experience:

• Bruising or bleeding
• Dizziness or lightheadedness.

After drinking the glucose drink, you may experience:

• Bloating
• Headache
• Nausea

How do you prepare for a glucose tolerance test?

To ensure the accuracy of your test:

• Inform your doctor if you are ill or taking any medications, as these can affect your OGTT test results.
• Do not eat or drink anything except water after 10.00pm on the night before the test.

What can you expect in a glucose tolerance test?

An OGTT is typically done in an outpatient clinic.

Estimated duration

An OGTT will take about 3 hours. This includes the waiting time between your blood tests.

Before the procedure

You must fast from food and drinks, except water, 8 hours before the test.

During the procedure

A nurse or phlebotomist will start by drawing a blood sample from your arm. Some people experience minor pain or discomfort during the blood test, but this usually does not last long.

After the first blood sample is taken, you will be given a bottle of glucose to drink. It will taste very sweet.

Your blood will be drawn several more times at regular intervals over the next 2 – 3 hours. In some cases, your blood will only be drawn once. You will be asked to remain in the waiting area and to continue fasting.

After the procedure

You should receive the results for your test in milligrams (mg) per decilitre (dL). If the levels in your OGTT result are:

• Below 140 mg/dL, your blood sugar is normal.
• Between 140 and 199 mg/dL, your glucose tolerance is impaired and you may have prediabetes.
  • If you have prediabetes, your doctor will advise you on lifestyle and dietary changes to prevent type 2 diabetes.
  • Generally, exercise and weight loss can help you to lower your risk for type 2 diabetes.
• 200 mg/dL or higher, you may have diabetes.
  • Your doctor may order additional tests to confirm your diagnosis. Meanwhile, they will advise you to make lifestyle and dietary changes. They may also prescribe medications to control your blood sugar.
  • If you are a pregnant mother with gestational diabetes, your blood sugar should return to normal after your baby is born. Nonetheless, eating a healthy diet and staying physically active can help to control gestational diabetes during pregnancy. These changes can also help to prevent type 2 diabetes in the future, as gestational diabetes increases your risk of diabetes after pregnancy.

Care and recovery after a glucose tolerance test

After your OGTT, you can return to your usual activities immediately. There is generally no recovery period needed.