If a patient needs to have a blood test, he or she needs to fast for 6-8 hours on the day of the test and can drink a small amount of water before the test, no more than 200 ml is appropriate. Blood sampling is an important and indispensable part of the physical examination. Fasting blood sampling refers to specimens taken on an empty stomach after 8 hours of fasting, usually in the morning before breakfast. Fasting blood can avoid the influence of dietary components and daytime physiological activities on the results of the examination, while the same time each day to take blood is also convenient for control. If the blood is not drawn on an empty stomach, the reference interval will lack comparability and can affect the interpretation of the results. In addition, drinking large amounts of water before the blood is drawn can dilute the blood and lead to errors in many of the test values. However, it does not mean that the patient cannot drink water, but it is generally appropriate to drink no more than 200 ml, but only plain water. Therefore, to ensure the accuracy of the blood draw and other tests, patients should avoid eating and drinking after 8:00 pm the night before the test.