What is glucose tolerance testing (GTT)?

Updated: May 10, 2021
  • Author: Jiun-Lih Jerry Lin, MBBS, MS(Orth); Chief Editor: Eric B Staros, MD  more...
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Glucose tolerance testing (GTT) evaluates the ability to regulate glucose metabolism. Plasma glucose reflects a combination of carbohydrate uptake from the gastrointestinal tract, hepatic glucose output, and peripheral glucose uptake. GTT is used when the serum glucose level alone is equivocal in diagnosing diabetes (ie, when the glucose level falls in the equivocal range). [2, 3]

For three days preceeding the GTT, the patient should have a diet containing approximately 150 g of carbohydrate daily. The patient should fast overnight for a minimum of 8 hours and a maximum of 16 hours. [10]  GTT is usually performed in the morning, and the patient should remain seated throughout the test. After baseline fasting plasma glucose testing, the glucose load is administered—either intravenously or, more commonly, orally—and plasma glucose is measured at specified intervals thereafter. In standard oral GTT, plasma glucose concentration is measured 2 hours after a 75 g oral glucose load; for gestational diabetes mellitus, an additional measurement may be made at 1 hour. The recommended glucose dose for a child is 1.75 g/kg body weight, with a maximum of 75 g. [10]

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