Routine preoperative checkup blood tests require fasting, and patients are required not to drink or eat after 10 o’clock in the evening, and start drawing blood around 6 o’clock in the morning. Because there are a lot of check items, if you drink or eat after the test results may be affected. For example, the routine blood test is to check the number of white blood cells, red blood cells, and platelets, which are needed to count the number of cells in the blood. Drinking water dilutes the blood, which may cause the normal white blood cell or red blood cell counts to drop, thus affecting the test results. Eating may affect the aminotransferases of the liver function and may also affect the blood glucose diagnosis, so fasting should be required before the blood is drawn.