(oral glucose tolerance test) the gold-standard diagnostic test for diabetes mellitus and the related condition, impaired glucose tolerance (IGT). After an overnight fast the blood-sugar level is measured (the fasting level). The patient is then given a drink containing 75 g glucose, and a further blood-sugar measurement is taken after two hours. 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 below 7.0 mmol/l and the two-hour level is between 7.0 mmol/l and 11.1 mmol/l. Many people with IGT progress to develop diabetes, but this can be prevented with adoption of a diabetic-type diet and weight loss (if overweight).