It is usually used for screening of suspected diabetic patients and those at high risk of diabetes. In undiagnosed patients, if fasting blood glucose is between 6.1 and 7 mmol/L or random postprandial blood glucose is between 7.8 and 11.1 mmol/L, an OGTT test should be performed. The specific practice of OGTT test is to take 75g of anhydrous glucose powder (dissolved in 200~300ml of water) orally after drawing blood once on fasting, and to draw blood at half hour, one hour, two hours and three hours respectively from the time of taking the first sip of sugar water. Preparatory measures: 1. Start from 7 to 9 am (must fast for 8 to 14 hours); 2. Since the OGTT test takes 3 hours, subjects should not drink tea or coffee, smoke or do strenuous exercise during the test to avoid affecting the results of blood glucose monitoring; 3. Drugs that may affect glucose tolerance, such as birth control pills, diuretics, phenytoin sodium, etc. If the fasting blood sugar of OGTT test is higher than 7.0 mmol/L, it should be changed to oral two-two bun meal to prevent aggravating the pancreatic islet burden and damaging the cells.