Called erythrocytosis, an elevated red blood cell count means that the bloodstream has too many red blood cells, according to Mayo Clinic. Erythrocytosis develops for various reasons, including the body's need to compensate for low oxygen levels due to a poorly performing heart or lungs.
Low oxygen levels at high altitudes may also cause an increase in red blood cells, advises Mayo Clinic. An increase in red blood cells may mean the patient's kidneys are releasing too much of the protein erythropoietin.
If the oxygen-carrying capacity of existing red blood cells decreases, the body may produce excess red blood cells, explains Mayo Clinic. Dehydration, carbon monoxide poisoning and congenital heart disease in adults can also lead to erythrocytosis.