Glucose tolerance test, also known as glucose tolerance test, is a laboratory test for the diagnosis of diabetes mellitus, and there are mainly two types of tests, intravenous and oral. Intravenous glucose tolerance test is suitable for special patients such as after gastrectomy or malabsorption syndrome, while oral glucose tolerance test (OGTT) is the most common clinical test. After taking a certain amount of glucose, the blood glucose concentration temporarily increases in normal people (usually not more than 8.9mmol/L), but it can return to normal fasting level within 2 hours. If the glucose metabolism is abnormal due to endocrine dysfunction and other factors, the blood glucose concentration may rise sharply after a certain amount of glucose is consumed, and it cannot return to the original concentration level within a short time. The oral glucose tolerance test (OGTT) should be performed in the early morning, with fasting blood drawn before 8:30 a.m. (blood glucose measurement), followed by drinking 250-300 ml of water containing 75 g of glucose within 5 minutes. Venous blood should be drawn for blood glucose measurement 1 hour, 2 hours and 3 hours after drinking sugar water. An insulin release test and/or C-peptide measurement should be done at the same time as the oral glucose tolerance test to evaluate the function of pancreatic islet cells. Indications for glucose tolerance test: 1.Clinical suspicion of diabetes mellitus, and the fasting blood glucose result alone cannot be determined. 2.When the blood glucose is higher than the normal range and does not meet the criteria for diagnosis of diabetes mellitus. 2.Diabetes mellitus has been diagnosed and a comprehensive understanding of peak glucose secretion, insulin secretion function and C-peptide is needed. 3.Diabetes caused by other reasons to identify, such as renal diabetes, trophoblastic diabetes, etc. Clinical significance of glucose tolerance test 1. Normal value: fasting 3.9-6.1mmol/L, 1 hour blood glucose rise to peak <11.1mmol/L, 2 hours fall <7.8mmol/L, 3 hours fall in fasting value. 2.Impaired glucose tolerance (IGT): oral glucose tolerance test (OGTT) 2-hour blood glucose level between 7.8 and 11.0mmol/L. 3.Confirmed diabetes: fasting blood glucose ≥ 7.0mmol/L or any time blood glucose ≥ 11.1mmol/L. In the early stage of diabetes, mainly postprandial blood glucose is elevated, and fasting blood glucose is mostly normal or high limit of normal. Oral glucose tolerance test (OGTT) is helpful for the diagnosis of occult diabetes, and in practical application, OGTT can also be simplified, that is, only fasting and 2 hours after sugar specimens are taken to determine the blood glucose value, and the 2 hours value is generally considered to be critical. Patients with a history of hypertension, dyslipidemia, obesity, coronary artery disease, a history of gestational diabetes and those over 40 years of age should be more vigilant and should take the initiative to do diabetes screening.