What is an oral glucose tolerance test (OGTT)?
An OGTT is a test in which blood is drawn from a vein to check for glucose levels (blood sugar) in the blood stream. Although a simple finger stick with a lancet may be used in your doctor’s office or at home to test your blood glucose level at a particular point in time, the most accurate way to see how your blood sugars rise and fall in response to glucose overload over longer periods of time is having the blood drawn from the vein in a lab.
Purpose of an OGTT
The OGTT is used to diagnose (or rule out) pre-diabetes, type 1 or type 2 diabetes, or gestational diabetes in pregnant women. If your doctor suspects you are at risk for diabetes, or have other signs or symptoms, he/she may order an oral glucose tolerance test.
Possible test results:
- You may have normal blood sugars (normoglycemia, or, euglycemia)
- Your blood sugar can be too high (hyperglycemia)
- Your blood sugar can be too low (hypoglycemia)
- Borderline condition, such as pre-diabetes
If your blood sugar levels are higher than normal, but lower than diabetic ranges, you will be classified as having impaired fasting glucose (IFG) or impaired glucose tolerance (IGT), both of which are used to diagnose pre-diabetes.
How is the test administered?
A small amount of blood is drawn from a vein as a baseline blood glucose level before you eat or drink anything (besides water.) To check and see how a person reacts to a glucose load, the lab will give you approximately 75 grams of glucose (a very sweet tasting beverage) to drink all at once. Blood sugar levels are then checked 2 hours after drinking the glucose by taking a second blood draw from the vein.
Does it hurt?
How much it hurts to have blood drawn from a vein depends on many things including the skill of the person drawing the blood, whether you have “good” veins, and your tolerance to pain. Drinking water 30 minutes before the blood draw can help make veins easier to access. If you are very sensitive to pain, ask about a topical cream to numb the area a few minutes before the blood is drawn.
What do the results of an OGTT show?
The following general guidelines are for normal blood sugar ranges in non-diabetics are from the American Diabetes Association.
These guidelines are not intended for “target blood sugar” ranges for people with diabetes. Young children, people newly diagnosed with diabetes, or who are beginning insulin pump therapy will have different target ranges set by their doctor.
Oral Glucose Tolerance Test Ranges
(except during pregnancy)
(per American Diabetes Association Guidelines)
2 Hours after drinking 75 grams of glucose
|Less than 140 mg/dL (7.8 mmol/L)||Normal glucose tolerance, not diabetic|
|From 140 to 200 mg/dL (7.8 to 11.1 mmol/L)||Impaired glucose tolerance (IGT), or Pre-diabetes|
|Over 200 mg/dL (11.1 mmol/L) or higher on more than one occasion||Type 2 Diabetes (an OGTT would never be given to a person suspected of having type 1 diabetes)|
Elevated Blood Glucose Levels Are A Warning Flag
Both IFG and impaired glucose tolerance (IGT) indicate pre-diabetes and are associated with an increased risk of developing type 2 diabetes. Lifestyle changes including weight loss and an exercise program, as well as possible oral medications are sometimes indicated.
There is no cure for type 2 (or type 1) diabetes, but pre-diabetes can often be completely reversed with proper medical intervention and changes in lifestyle.