Glucose tolerance test (oral glucose tolerance test, OGTT) the gold-standard diagnostic test for diabetes mellitus and for impaired glucose tolerance (IGT) and impaired fasting glucose (IFG) (known as prediabetes or abnormal glucose regulation). The blood glucose level is measured after an overnight fast (water only) lasting at least 12 hours, and a second blood glucose level is measured 2 hours after a drink containing 75 g of glucose. The current WHO criteria for normal glucose tolerance is a fasting blood glucose level below 6.1 mmol/l and a two-hour blood glucose level below 7.8 mmol/L. Diabetes is diagnosed if the fasting level is above 7.0 mmol/l and/or the two-hour level is above 11.1 mmol/l. IGT is diagnosed when the fasting level is less than 7.0 mmol/l and the two-hour level is 7.0-11.1 mmol/l, and IFG when the fasting level is 6.0-7.0 mmol/l but the two-hour level is less than 7.8 mmol/L. People with IGT and IFG have a high risk of progression to type 2 diabetes over time.

Reference: Oxford Concise Colour Medical Dictionary (7th Edition, Kindle Edition) Oxford University Press 2020 ISBN-13: 978-0198836629, p1553