Glucose tolerance test

A glucose tolerance test in medical practice is the administration of glucose 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. The glucose is most often given orally so the common test is technically an oral glucose tolerance test (OGTT). The test may be performed as part of a panel of tests, such as the comprehensive metabolic panel.

Indications for OGTT:

In non pregnant adults an OGTT is recommended,

• For all patients with impaired fasting glucose.
• When results of the recommended diagnostic tests (FPG and RPG) fail to determine the diagnosis in an individual patient (E.g. equivocal or borderline results).

Protocol for the OGTT

1. Preparation of the patient:

• Three days unrestricted, carbohydrate rich diet and activity.
• No medication on the day of the test.
• 8 to 14 h fasting
• for paediatric patients: Children > 6 years: 8 -10 hr fast,Children <> 6.1 & <7.0>7.8 >7.0 >11.1
(mg/dl) <110>110 & <126>140 >126 >200

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 that are said to be characteristic. Using a glucose tolerance in this context resembles use of a Rorschach test in that it is often used to support a diagnosis that the patient and doctor are already reaching agreement on based on other evidence, but it is inadequate by itself to confirm or refute the diagnosis (unlike its use for diabetes).
When the glucose is given intravenously it is termed an intravenous glucose tolerance test (IVGTT). This has been used in the investigation of early insulin secretion abnormalities in prediabetic states.