Glucose tolerance tests (GTTs) are used to evaluate the response to a carbohydrate challenge throughout a 3- to 5-hour period. When a glucose load is presented, the normal individual's blood insulin level rises in response to it, with peak levels occurring 30 to 60 minutes after the carbohydrate challenge. Blood glucose levels, although elevated immediately after the carbohydrate challenge, return to normal fasting levels 2 to 3 hours later. For individuals in whom abnormal hypoglycemia or gastrointestinal malabsorption is suspected, the test may be extended to a 5-hour period.12, 13, 14
Several methods can be used to perform a glucose tolerance test. The oral, IV, and cortisone glucose tolerance tests are discussed in this section. Tolerance tests also may be performed for pentose, lactose, galactose, and D-xylose.