It is recommended not to drink water before the blood draw, especially not in large quantities, because drinking water may have an interfering effect on the results of the blood test. For example, if a large amount of water is drunk before the blood test, the water is absorbed into the body, which causes an increase in blood volume and blood dilution, so that the density of various blood cells will appear to decrease. It may also lead to a decrease in the concentration of hemoglobin, which may interfere with the normal results and may even cause the illusion of having anemia. If you drink water before the blood test, in addition to various serological tests will also produce interference, like the titer of various antibodies, after drinking a lot of water, causing the dilution of blood. The titer of various antibodies will drop, which will also have an impact on the judgment of the results. Therefore, it is recommended not to drink water before the blood draw.