High mean red blood cell content refers to a higher than normal number of red blood cells per unit volume of blood. It is generally considered to be a high average red blood cell content when the red blood cells are >6 million/mm^3 in adult males and >5.5 million/mm^3 in adult females after multiple examinations. Relative increase refers to a decrease in plasma volume, resulting in a relative increase in red blood cell volume, seen in vomiting, diarrhea, profuse sweating, massive burns, uremia, hyperalgesia, thyroid crisis, diabetic ketoacidosis, etc. Absolute increase, including true erythrocytosis and secondary erythrocytosis. Secondary erythrocytosis is seen in residents of highland areas, severe cardiopulmonary disorders, abnormal hemoglobinopathies with low oxygen-carrying capacity, etc. It is due to low oxygen saturation, which causes a compensatory increase in erythropoietin. Secondary erythropoiesis can also be caused by a non-compensatory increase in erythropoietin due to kidney cancer, hepatocellular carcinoma, uterine fibroids, ovarian cancer, polycystic kidney, etc.