Drinking small amounts of water does not usually have an effect on the results of a blood draw, but drinking large amounts of water may affect the counts of red blood cells, hemoglobin, and so on. Drinking small amounts of water before a blood test will not affect the normal test results. However, drinking large amounts of water prior to the blood draw may cause dilution of the blood, which in turn may affect the counts of hemoglobin, red blood cells, white blood cells, and platelets, resulting in measurements that are lower than the true values. In addition, subjects are required to keep fasting for more than 8 hours before blood sampling to avoid changes in liver function, blood glucose, blood lipids and other biochemical indexes caused by eating, which may lead to inconsistency between the measured value and the true value. Therefore, it is recommended to keep fasting for at least 8 hours before blood sampling, and can drink a small amount of water, but should not drink a lot of water.