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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 hypoglycaemia. Most commonly the glucose is given orally so the common test is technically an oral glucose tolerance test (OGTT).

Preparation and cautions

The patient is instructed not to restrict carbohydrate intake in the days or weeks before the test. The test should not be done during an illness, as results may not reflect the patient's glucose metabolism when healthy.

Procedure for OGTT

The patient should have been fasting for the previous 8-14 hours.

A zero time (baseline) blood sample is drawn.

The patient is given a glucose solution to drink. The standard dose since the late 1970's has been 1.75 grams of glucose per kg of body weight, to a maximum dose of 75 g. It should be drunk within 5 minutes. Prior to 1975 a dose of 100 g was often used. Blood is drawn at intervals for measurement of glucose (blood sugar), and sometimes insulin levels. The intervals and number of samples vary according to the purpose of the test. For simple diabetes screening, the most important sample is the 2 hour sample and the 0 and 2 hour samples may the only ones collected.

Interpretation of OGTT results

Fasting glucose should be below 111 mg/dl. Fasting levels between 110 and 126 are borderline (?impaired fasting glucose?), and fasting levels repeatedly above 125 are diagnostic of diabetes. The 2 hour glucose should be below 140 mg/dl. Levels between 140 and 200 indicate ?impaired glucose tolerance.? Glucose above 200 at 2 hours confirms a diagnosis of diabetes.

Whilst impaired glucose tolerance is an asymptomatic condition, it carries an increased risk of large vessel disease.

In Europe, blood glucose is measured in mmol/l. 1 mmol/l=18 mg/dl. For the values above, 110 mg/dl=6.2 mmol/l, and 125=7, 140=7.8, 200=11.1)

Variations

A standard 2 hour OGTT is sufficient to diagnose or exclude diabetes. 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.

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