The oral glucose tolerance test (OGTT) measures the body's ability to use a type of sugar, called glucose, that is the body's main source of energy. An OGTT can be used to diagnose prediabetes and diabetes. An OGTT is most commonly done to check for diabetes that occurs with pregnancy (gestational diabetes).
The oral glucose tolerance test (OGTT) is done to:
To prepare for the glucose tolerance diagnostic test:
The glucose tolerance diagnostic test may take up to 4 hours. Since activity can interfere with test results, you will be asked to sit quietly during the entire test. Do not eat during the test. You may drink only water during this time.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, or how it will be done. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?).
On the day of testing, the following steps will be done:
The health professional taking a sample of your blood will:
You may find it hard to drink the extremely sweet glucose liquid. Some people feel sick after drinking the glucose liquid and may vomit. Vomiting may prevent you from completing the test on that day.
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
You may feel faint from having several blood samples taken in one day. But the amount of blood taken will not cause significant blood loss or anemia.
Some people's blood glucose levels drop very low toward the end of the test. But some people feel like their sugar levels are low, when their levels actually are not low. Symptoms of low blood glucose include weakness, hunger, sweating, and feeling nervous or restless. If you develop these symptoms during the test, you may have your sugar level checked quickly with a glucose meter. If your level is very low, the test will be stopped.
There is very little risk of a problem from having blood drawn from a vein.
The oral glucose tolerance test (OGTT) measures the body's ability to use a type of sugar, called glucose, that is the body's main source of energy.
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
But with screening for gestational diabetes, the American Diabetes Association has recommended specific glucose values be used for diagnosis. If any of your glucose values are higher than what is listed in the table, you will be diagnosed with gestational diabetes.
|75 g of glucose||Fasting:|
Less than 95 mg/dL or 5.3 mmol/L
Less than 180 mg/dL or 10.0 mmol/L
Less than 153 mg/dL or 8.5 mmol/L
You have prediabetes if the results of your oral glucose tolerance test are 140 to 199 mg/dL (2 hours after the beginning of the test).
High glucose levels may be caused by:
Low glucose levels may be caused by:
Many conditions can change blood glucose levels. Your doctor will discuss any significant abnormal results with you in relation to your symptoms and past health.
Reasons you may not be able to have the test or why the results may not be helpful include:
If you are diagnosed with gestational diabetes, you have a higher risk of developing diabetes later in life. The American Diabetes Association recommends screening for diabetes and prediabetes every 3 years.2
- American Diabetes Association (2012). Diagnosis and classification of diabetes mellitus. Diabetes Care, 35(Suppl 1): S64–S71.
- American Diabetes Association (2013). Standards of medical care in diabetes—2013. Diabetes Care, 36(Suppl 1): S11–S66.
Other Works Consulted
- Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.
|Primary Medical Reviewer||E. Gregory Thompson, MD - Internal Medicine|
|Specialist Medical Reviewer||Alan C. Dalkin, MD - Endocrinology|
|Last Revised||July 5, 2011|
Last Revised: July 5, 2011
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