The oral glucose tolerance test (OGTT) aims to measure the body's ability to use glucose (blood sugar) for energy. The test is commonly used to diagnose medical conditions where the body cannot use glucose properly, such as prediabetes (where blood sugar is higher than normal, but not high enough for diabetes), diabetes, and gestational diabetes (diabetes that occurs during pregnancy).
When used to screen for gestational diabetes, this test is usually conducted during Week 24 to Week 28 of pregnancy. A woman may be at increased risk of developing diabetes during pregnancy if she:
- had previous gestational diabetes
- previously gave birth to a heavy baby (more than 4 kg or 8.8 lbs)
- is younger than 25 years of age and was overweight before pregnancy
The test may also be useful to help detect other medical conditions that may affect blood sugar. Your doctor will determine if this test is required for you.
Risks and precautions
It is important that you understand all the risks of complications and side effects of the test, and what you or your doctor can do to avoid them. Make sure that your doctor is aware of all your concerns.
Glucose tolerance test is usually a straightforward and safe test. However, there are some risks of complications or side effects, including:
- bruising at the needle site
- excess bleeding
- inflammation of the veins
Your blood sugar may drop very low at the end of the test. You may feel weak, hungry, sweaty, nervous, and restless. You should tell your health care provider when you experience these symptoms and the test may have to be stopped.
Before the test
It is important that you fully understand what the test involves beforehand. Ask your doctor to explain the risks, benefits, and drawbacks of the test, and don't be shy to probe further until you are comfortable with your doctor's responses.
If you are undergoing this test to screen for gestational diabetes, you do not need to do any special preparation.
If you are undergoing this test to screen for diabetes or prediabetes, you should:
- eat a diet that contains a specific amount of carbohydrates, as instructed by your doctor, starting 3 days before the test
- do not eat, drink, smoke, or exercise at least 8 hours before the test
If you are taking any prescription or over-the-counter (non-prescription) medications, supplements, or herbal products, make sure you inform your doctor or pharmacist. Ask them whether it is necessary for you to stop taking any of these medications and products before the test. It is also important to tell them if you have allergies to certain medications or have certain medical conditions.