A glucose tolerance test is a medical test in which glucose is given and blood samples taken afterward to determine how quickly it is cleared from the blood. The test is usually used to test for diabetes, insulin resistance, and sometimes reactive hypoglycemia and acromegaly, or rarer disorders of carbohydrate metabolism. In the most commonly performed version of the test, an oral glucose tolerance test
(OGTT), a standard dose of glucose is ingested by mouth and blood
levels are checked two hours later. Many variations of the GTT have been
devised over the years for various purposes, with different standard
doses of glucose, different routes of administration, different
intervals and durations of sampling, and various substances measured in
addition to blood glucose.
Since the 1970s, the World Health Organization and other organizations interested in diabetes agreed on a standard dose and duration.
Impaired glucose tolerance is often associated with insulin resistance and is often seen in Polycystic Ovarian Syndrome.
A standard 2 hour OGTT is sufficient to diagnose or exclude all forms of diabetes mellitus at all but the earliest stages of development. Longer tests have been used for a variety of other purposes, such as detecting reactive hypoglycemia or defining subsets of hypothalamic obesity. Insulin levels are sometimes measured to detect insulin resistance or deficiency.
The OGTT is of limited value in the diagnosis of reactive hypoglycemia, since normal levels do not preclude the diagnosis, abnormal levels do not prove that the patient's other symptoms are related to a demonstrated atypical OGTT, and many people without symptoms of reactive hypoglycemia may have the late low glucoses.
When the glucose is given intravenously it is termed an intravenous glucose tolerance test (IVGTT) or intravenous glucose challenge test (IVGCT). This has been used in the investigation of early insulin secretion abnormalities in prediabetic states.
Since the 1970s, the World Health Organization and other organizations interested in diabetes agreed on a standard dose and duration.
| Glucose levels | NORMAL | impaired fasting glycaemia (IFG) | impaired glucose tolerance (IGT) | Diabetes Mellitus (DM) | ||||
|---|---|---|---|---|---|---|---|---|
| Venous Plasma | Fasting | 2hrs | Fasting | 2hrs | Fasting | 2hrs | Fasting | 2hrs |
| (mmol/L) | <6.1 | <7.8 | > 6.1 & <7.0 | <7.8 | <7.0 | >7.8 | >7.0 | >11.1 |
| (mg/dL) | <110 | <140 | >110 & <126 | <140 | <126 | >140 | >126 | >200 |
A standard 2 hour OGTT is sufficient to diagnose or exclude all forms of diabetes mellitus at all but the earliest stages of development. Longer tests have been used for a variety of other purposes, such as detecting reactive hypoglycemia or defining subsets of hypothalamic obesity. Insulin levels are sometimes measured to detect insulin resistance or deficiency.
The OGTT is of limited value in the diagnosis of reactive hypoglycemia, since normal levels do not preclude the diagnosis, abnormal levels do not prove that the patient's other symptoms are related to a demonstrated atypical OGTT, and many people without symptoms of reactive hypoglycemia may have the late low glucoses.
When the glucose is given intravenously it is termed an intravenous glucose tolerance test (IVGTT) or intravenous glucose challenge test (IVGCT). This has been used in the investigation of early insulin secretion abnormalities in prediabetic states.
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