Erythrocytosis is a relative or absolute increase in the number of circulating RBC, resulting in a PCV increased above reference ranges. Polycythemia is frequently used synonymously with erythrocytosis; however, polycythemia may imply leukocytosis and thrombocytosis, as well as erythrocytosis.

Causes and Risk factors of Erythrocytosis include:
1. Problems with the bone marrow
Bone marrow is found in the centre of most bones and normally produces all red blood cells and platelets and most white blood cells. When this process breaks down, production of cells is no longer orderly and conditions such as polycythemia can result. Although the exact cause of polycythemia vera is unknown, researchers have found that a specific gene mutation is associated with almost all people with this condition.

2. A condition affecting men in their 60s
It is more common in men than in women. The condition is rarely seen in people under age 40, but can also affect babies.

3. Secondary polycythemia
A high content of red blood cells builds up in response to low oxygen concentration in the air. Because there's less oxygen in the blood, the body attempts to overcome the lack by making more red blood cells. It doesn't stop, however, and keeps producing them until there are too many. The bone marrow can also be overstimulated by testosterone replacement therapy.

4. High altitudes
It also may occur in people with chronic lung conditions and certain kidney tumours and cysts. Heavy smoking is associated with an increase in carbon monoxide in the blood and may also lead to higher red cell and hemoglobin levels. Low blood oxygen levels (hypoxia) due to congenital heart disease also appear to be a factor in the development of polycythemia.