Glucose Tolerance Test

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What is the glucose tolerance test?

Though not routinely used anymore, the oral glucose tolerance test (OGTT) has been considered to be the gold standard for making the diagnosis of type 2 diabetes. It is still commonly used during pregnancy for diagnosing gestational diabetes. With an oral glucose tolerance test, the person fasts overnight (at least 8 hours, but not more than 16 hours). The next morning, the fasting plasma glucose is tested. After this test, the person receives 75 grams of glucose. There are several methods employed by obstetricians to do this test, but the one described here is standard. Usually, the glucose is in a sweet-tasting liquid that the person drinks. Blood samples are taken up to four times at different time points after consumption of the sugar to measure the blood glucose.

How reliable is the glucose tolerance test?

For the glucose tolerance test to give reliable results, the person must be in good health (not have any other illnesses, not even a common cold). Also, the person should be normally active (not lying down, for example, as an inpatient in a hospital) and should not be taking medicines that could affect the blood glucose. In preparation for the oral glucose tolerance test, the person should eat and drink as they normally would. The morning of the test, the person should not smoke or consume caffeine.

What does the glucose tolerance test measure?

The classic oral glucose tolerance test measures blood glucose levels five times over a period of three hours. Some physicians simply take a baseline blood sample followed by a sample two hours after drinking the glucose solution. In a person without diabetes, the glucose levels rise and then fall quickly. In someone with diabetes, glucose levels rise higher than normal and fail to come back down as fast.

People with glucose levels between normal and diabetic levels have so-called impaired glucose tolerance (IGT). People with impaired glucose tolerance do not have diabetes.

Each year, 1% to 5% of people whose test results show impaired glucose tolerance actually develop diabetes. Weight loss and exercise may help people with impaired glucose tolerance return their glucose levels to normal. In addition, some physicians advocate the use of medications, such as metformin (Glucophage), to help prevent/delay the onset of overt diabetes. Studies have shown that impaired glucose tolerance itself may be a risk factor for the development of heart disease, and whether impaired glucose tolerance turns out to be an entity that deserves treatment itself is something that physicians are currently debating.

What is the preparation for a glucose tolerance test?

As mentioned previously, preparation for the oral glucose tolerance test involves fasting overnight (from 8 to 16 hours) and participating normally in activities of daily living. The individual should eat and drink as they normally do prior to the test. The morning of the test, the person should not consume caffeine or smoke.

10 Diabetes Questions to Ask Your Doctor

Note: We recommend you use this page as a reference for your consultation with your doctor.

Medically Reviewed by: Melissa Conrad Stöppler, MD

1. Should I check my blood sugar levels at home with a glucose monitor? How often should I check them?

2. What are my goals regarding blood sugar levels?

3. What are the warning signs or symptoms that my blood sugars are too high? What do I do if my blood sugars are too high?

4. What are the warning signs or symptoms that my blood sugars are too low? What do I do if my blood sugars are too low?

5. How can I change my lifestyle and diet in a way that will be healthy?

6. What are the side effects of my medications/insulin?

7. Will I always need medications/insulin? How will you evaluate whether these medications are the best treatment for me?

8. What are the long-term complications of diabetes, and how can I avoid them?

9. How do other factors such as high cholesterol and high blood pressure affect me if I have diabetes?

10. How often should I be seeing my doctor to optimize my diabetes management?

MedicineNet Reminder: Establishing an accurate diagnosis is key to proper treatments. You are the most important person in this process by accurately describing to your doctor the character, location, duration, and time of onset of your symptoms. You should also inform your doctor about vitamins, herbs, and medications you are taking. For example, long-term use of certain vitamins and non-prescription medications may be the cause of your abnormal liver tests; magnesium-containing antacids and supplements may be causing your diarrhea; certain blood pressure pills can be the reason for your constipation.


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