There are many causes of an as shown in Table 1. These causes may be subdivided into whether there is a 'true' (or absolute) erythrocytosis due to an increase in red cells, or an 'apparent' (when the red cells are not increased but are instead more concentrated). The distinction between an absolute and an apparent is made based upon the result of an investigation known as the red cell mass and plasma volume test (see below). An absolute is then further categorised into primary or secondary causes according to whether the abnormality is within (primary) or outside (secondary) the red cell itself. Rarely an absolute can be inherited. The only cause of a primary inherited erythrocytosis that has been described so far is a mutation in the receptor that binds to the cytokine, or hormone, that stimulates the synthesis of red cells – the process called erythropoiesis. This cytokine is known as erythropoietin and its site of manufacture is within specialised oxygen-sensitive cells within the kidneys. The mutation in the r removes the portion of the receptor that would normally dampen the positive action of erythropoietin binding to the receptor. Thus the overall result of this mutation is an unchecked increase in red cell production, causing an erythrocytosis. Inherited causes of a secondary s include a type (variant) of haemoglobin that binds more tightly to oxygen than normal). This higher affinity for oxygen will result in less oxygen being delivered to the tissues of the body. The body compensates for this by increasing the total quantity of haemoglobin (and thus red cells). Another rare cause of inherited in some individuals is a higher-than-normal level of erythropoietin. Finally some people inherit a lower-than-normal concentration of a compound called 2,3 diphosphoglycerate [2,3,DPG]. This compound reduces the affinity of haemoglobin for oxygen and the effect of this abnormality is a compensatory erythrocytosis. Much more commonly is not inherited but occurs as a result of a disease process in the individual. These acquired erythrocytoses most frequently arise from a stimulus to increase red cell production arising from outside the bone marrow (secondary erythrocytosis). Less often it occurs due to a primary abnormality of the red cells in the bone marrow itself, otherwise known as polycythaemia rubra vera (PV). Secondary acquired erythrocytoses fall into two main groups. First is reduced oxygen supply to the kidney, perhaps from blockage to a kidney artery or because the person's general oxygen state is reduced through lung disease. This causes a reflex increase in the production of erythropoietin and hence an increase in red cell production. In the other group of secondary erythrocytoses there is enhanced erythropoietin production, often caused by a tumour, as listed in Table 1. | 
