Variations
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 (1) normal levels do not preclude the diagnosis, (2) abnormal levels do not prove that the patient's other symptoms are related to a demonstrated atypical OGTT, and (3) 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.
Read more about this topic: Glucose Tolerance Test
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