People with excess red blood cells cannot donate blood. Excessive red blood cells may be caused by the concentration of blood due to the loss of a large amount of water in the body as a result of severe vomiting, diarrhea, or extensive burns, or it may be caused by the occurrence of chronic myeloproliferative neoplasms such as erythropoietic dyscrasia. 1. Patients with severe vomiting, diarrhea, or extensive burns will have a decrease in total blood circulation due to a large amount of water loss. Donating blood in this case may lead to a further decrease in the amount of circulating blood, which may result in hypovolemic shock, and therefore should not be used for donating blood. 2. When chronic myeloproliferative neoplasms such as true erythrocytosis occur, the red blood cells in the patient’s blood are the products of abnormal hematopoiesis of the bone marrow, which usually can not be used for clinical treatment, so this group of people also can not donate blood.