Glucose Tolerance Test

A glucose tolerance test measures how the body handles a defined glucose load over time. In the oral glucose tolerance test, a person consumes a standardized glucose drink after an appropriate preparation period, and blood glucose is measured before and at one or more specified times afterward. The resulting pattern reflects the combined effects of glucose absorption, insulin secretion, insulin action, and glucose disposal .¹

A commonly used adult protocol outside pregnancy uses 75 grams of oral glucose, but the exact sampling schedule and diagnostic interpretation depend on the clinical purpose and guideline. Different protocols are used for diagnosing diabetes, impaired glucose regulation, and gestational diabetes. For that reason, a glucose value cannot be interpreted correctly without knowing the test protocol and timing .¹

The test differs from a fasting plasma glucose measurement, which examines glucose at a single fasting time point. It also differs from HbA1c, which reflects longer-term exposure to glucose. A glucose tolerance test specifically challenges glucose regulation and observes the response, making it useful when fasting measurements alone do not capture how efficiently glucose is handled after a carbohydrate load .¹