HCT is the erythrocyte specific volume, which is the volume of red blood cells in the peripheral blood as a percentage of the total blood volume. Normal erythrocyte specific volume in men shows 40%-50% and in adult women is 37%-48%. An increased erythrocyte volume can be seen in a variety of situations: the first is when a person has a high erythrocyte volume after severe diarrhea and profuse sweating, which increases the number of red blood cells per unit volume due to blood volume concentration. The second condition is an absolute increase in the number of red blood cells, such as chronic systemic diseases, such as chronic obstructive pulmonary disease, congenital cyanotic heart disease, etc. Due to long-term hypoxia, the concentration of erythropoietin in the body increases, stimulating the proliferation of red blood cells, resulting in a high erythrocyte volume, and then in patients with true erythrocytosis, due to an abnormal increase in the number of red blood cells, the erythrocyte volume will also appear abnormally high.