Fasting is generally defined as not eating for 8 to 14 hours. That is to say, if you need a medical examination at 8 am on a certain day, you should eat the night before the blood draw, around 6 pm. After that, do not eat late night snacks, of course, the next morning breakfast, but also after the physical examination. Also, be careful not to fast for too long, so as not to cause inaccuracies in the detection of blood sugar and other indicators. Different tests have different requirements for drinking water. There are many varieties of drinks, and our discussion here refers to clear water: Need to drink a lot: Before many ultrasound examinations, you need to drink water, such as urological ultrasound and gynecological ultrasound. These examinations also require holding urine, the purpose of which is to make the bladder expand and push the organ to be examined as far as possible to the abdominal wall to facilitate the examination. You must not drink: painless (anesthesia) gastroscopy will never allow you to drink water, as choking and asphyxiation are easily caused by anesthesia. For other examinations, you may drink a little water. Because the rate of water loss varies from person to person, it is better to drink as much as you can without feeling thirsty. Try not to drink any other drinks. For example, sugary fruit juices, milk, tea, will directly change the laboratory indicators within the blood; and like coffee, even black coffee, will change the changes in the blood hormone levels and affect the blood biochemical indicators. And patients with chronic diseases do not need to be afraid to even take drugs because of the fear of drinking water. As long as it is taken with water there is no effect on the above mentioned tests, because the water stays in the stomach for a very short time. In order to maintain the stability of their indicators and to better reflect their general condition, they should not stop taking their medication hastily. Which tests require fasting 1. Tests that require direct observation of the digestive tract: (1) Barium meal: It is an X-ray test to observe the digestive tract, which requires taking barium sulfate contrast to observe the wall of the digestive tract. Under the X-ray, the barium meal is coated on the surface of the digestive tract and is bright white, and if there is food to cover it, it will be shown as a black shadow protruding from the surface of the digestive tract. (2) Gastroscopy: During gastroscopy, enteroscopy and other endoscopic examinations of the GI tract, a thin mirror is extended into the GI tract for observation, so undigested food is a roadblock on its way forward. 2. Examination of the dorsal organs of the digestive tract: Pancreatic ultrasonography: The pancreas is located at the back of the intestine, in the middle of the abdominal cavity, and can neither be observed from behind nor close to the belly. Instead, ultrasonography needs to pass through the belly and intestine to reach the pancreas. After eating, there will be a lot of air swallowed along with the food, and food and air ambush like mountain bandits in the path of ultrasound advancement, losing a lot of sound waves. In this way, ultrasound cannot reach the dorsal side through the intestine and interferes with the observation of the pancreas. 3. Examination of the organs that change after eating: Gallbladder: Bile is secreted by the liver and stored by the gallbladder. Bile assumes an important role in our digestion of fats. During feeding, the gallbladder contracts and bile is squeezed into the intestine. Therefore, when you eat, the gallbladder contracts and it is difficult to observe. Therefore, fasting is required before all relevant examinations. 4, certain biochemical tests: fasting can avoid the ingredients in the food interfering with the results of the relevant blood laboratory indicators, such as blood sugar, blood lipids and other indicators, which are bound to rise after eating; and some indicators such as insulin, which can cause fluctuations due to eating and interfere with the test results. 5, need to stimulate the examination of the digestive tract: stimulate the throat easily cause nausea, such as laryngoscopy, gastroscopy, bronchoscopy, transesophageal ultrasonography and other examinations, must be fasting. In addition, some imaging tests, such as enhanced CT and enhanced MRI, require the injection of contrast agents, which may cause allergies and subsequent vomiting, so fasting is recommended for safety reasons. In short, on the day of the physical examination, do not eat breakfast, drink a small amount of water before the blood draw, and bring a bottle with you so that you can drink immediately when you need to drink a lot of water to hold urine for certain examinations.