The glucose tolerance test (GTT) is used to identify abnormal glucose metabolism in diabetes mellitus and certain other diseases.
In the usual procedure, a patient who has fasted 18 to 24 hours drinks a concentrated solution of
glucose, and glucose
concentration is measured in blood and urine samples taken at one-half, one, two, and three hours after ingestion. In healthy individuals, glucose concentration rises to about twice the normal level within the first hour and returns to normal within two hours; little or no glucose is excreted in the urine. In diabetics, blood glucose rises to a much higher level, the return to normal levels is delayed three hours or more, and a large amount of glucose is excreted in the urine. The diabetic's response is due to lack of insulin, a hormone required for the removal of glucose from the blood. There is also a GTT for hypoglycemia, in which samples are taken after 5 hours. In a hypoglycemic, the blood sugar level is below normal, and sugar is absent in urine.