Glucose tolerance test

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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.[1] The test is usually used to test for diabetes, insulin resistance, and sometimes reactive hypoglycemia 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.

Contents

Standard OGTT

Since the 1970s, the World Health Organization and other organizations interested in diabetes agreed on a standard dose and duration.

Preparation

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. A full adult dose should not be given to a person weighing less than 43 kg (94 lb), or exaggerated glucoses may produce a false positive result. Usually the OGTT is performed in the morning as glucose tolerance can exhibit a diurnal rhythm with a significant decrease in the afternoon. The patient is instructed to fast (water is allowed) for 8–12 hours prior to the tests.

Procedure

Dose of glucose and variations

  • In the US, dosing is by weight, and since the late 1970s has been 1.75 grams of glucose per kilogram of body weight, to a maximum dose of 75 g. Prior to 1975 a dose of 100 g was often used.
  • The WHO recommendation is for a 75g oral dose in all adults: the dose is adjusted for weight only in children.[2] The dose should be drunk within 5 minutes.
  • A variant is often used in pregnancy to screen for gestational diabetes, with a screening test of 50 grams over one hour. If elevated, this is followed with a test of 100 grams over three hours.

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