A day without water does not usually result in a significant increase in hemoglobin. Hemoglobin is an important component of red blood cells, which can combine with oxygen to transport oxygen and carbon dioxide; hemoglobin is also an important indicator of the degree of anemia, and hemoglobin concentration is very stable over a certain period of time. When you don’t drink water for a day, your blood will be concentrated to a certain degree, and your hematocrit may increase slightly, but your red blood cell count and hemoglobin concentration will generally not rise significantly. Even if there is a slight fluctuation, it will be within the normal range. The normal value of hemoglobin is 120~160g/L for men and 110~150g/L for women. Therefore, you can eat and drink before the blood test, and it will not affect the counts of hemoglobin, red blood cells, white blood cells and platelets. Most of the mild elevation of hemoglobin is physiological, such as plateau reaction, strenuous activities, excessive fear of stimulation. If hemoglobin is significantly elevated, it may be congenital heart disease, pulmonary heart disease, true erythrocytosis, etc. Further examination is needed to clarify the cause.