Blood sugar generally refers to the glucose content in blood, and the commonly used clinical blood sugar value is serum blood sugar value, which refers to the blood sugar value measured by drawing venous blood. Generally fasting blood glucose is 3.9-6.1mmol/L, and postprandial blood glucose has different normal values according to the time: 1. Fasting blood glucose: normal value 3.9-6.1mmol/L, fasting blood glucose measurement requires fasting water for 8 hours before measurement, and pressing the measured part for 3-5 minutes after measurement to avoid bleeding. Fasting blood glucose in the range of 6.1-7.0mmol/L is called impaired fasting blood glucose. Fasting blood glucose >7.0mmol/L can be diagnosed as diabetes mellitus. The decrease of fasting blood sugar can be seen in physiological decrease, excessive insulin, hepatic hypoglycemia, etc.; 2. Postprandial blood sugar: 1 hour postprandial blood sugar should be <11.1mmol/L, 2 hours postprandial blood sugar should be 3.9-7.8mmol/L. The diagnosis of diabetes requires the use of glucose tolerance test, 2 hours postprandial blood sugar in 7.8-11.1mmol/L is called impaired glucose tolerance. A 2-hour postprandial blood glucose at >11.1mmol/L can be diagnosed as diabetes mellitus. If abnormal blood glucose is found on a single occasion, it is recommended to test blood glucose again on another day to avoid dietary effects, etc. Impaired fasting glucose and hypoglycemic tolerance are both prediabetes, and they are important high-risk groups for diabetes. They need to actively adjust their lifestyles, monitor their blood glucose regularly, and perform glucose tolerance tests to clarify the presence of diabetes if necessary.