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In the oral glucose tolerance test the patient, after fasting for 15 hours, drinks 75 g of glucose in 300 ml of water, over 5 minutes. Blood glucose is measured before the drink and after 30, 60, 90 and 120 minutes. Urine is collected before the drink, and after 60 and 120 minutes. Before the test, for a period of 6 hours, the patient should avoid smoking and drinking coffee.
There is a normal glucose tolerance if the venous plasma value is less than 7.8 mmol/l after the 2 hour period, i.e. diabetes and impaired glucose tolerance are excluded.
If after 2 hours after the glucose load the value is >=7.8 and <= 11 mmol/l, then there is impaired glucose tolerance.
If glucose is greater than or equal to 11.1 mmol/l, diagnose diabetes mellitus, if symptoms are present; in asymptomatic patients, a further value greater than or equal to 11.1 mmol/l during the oral glucose tolerance test is required.
If the blood glucose reaches its highest level at 30 min and then falls below normal levels at 60 and 90 min then this is a lag curve. This profile is due to rapid glucose absorption followed by a burst of insulin production which over-compensates, resulting in hypoglycaemia.
If the blood glucose rises only a small amount, this is called a flat profile.