Drinking a small amount of water before drawing blood has little effect, but after drinking a large amount of water, it will have a significant impact on the results of the test. For example, when doing routine blood tests, if you drink a lot of water before the blood draw, water absorption will cause dilution of blood volume, blood volume increases, so that the density of blood cells decreased, for example, the density of red blood cells decreased, while the concentration of hemoglobin also decreased, which is easy to cause the patient to have anemic conditions of the illusion of interfering with the doctor’s judgment of the disease. After drinking water, it is easy to have a significant impact on the titer of various antibodies in serology, for example, when checking the titer of various viral antibodies, the titer of the antibody may decrease after drinking water, which can easily lead to a misjudgment and create the illusion that the titer of the antibody of the patient has decreased. Therefore, it is better not to drink water, especially a lot of water, before the blood test.