An erythrocytosis is present when the red cell mass is greater than or equal to 125% of the predicted value for an individuals body mass and sex. The erythrocytosis is reflected by the fact that the haemoglobin and haematocrit are also usually raised. By definition it is congenital if it is present from birth. In such persons the erythrocytosis is usually detected at a young age. There is frequently a family history in keeping with the inherited nature of the defect.
Congenital erythrocytoses can be primary or secondary. A primary erythrocytosis occurs when there is an intrinsic defect in the erythroid cells in the bone marrow. In these cases erythropoietin (EPO) levels with be below normal.
A secondary erythrocytosis is present when EPO is produced from some source and then drives red cell production resulting in an erythrocytosis. In this situation EPO levels will be normal (which would be inappropriate for a raised haematocrit) or elevated above the normal range. Alterations of the genes in the oxygen sensing pathway can cause such a secondary congenital erythrocytosis. However, other congenital lesions such as, high oxygen affinity haemoglobins and bisphosphoglycerate mutase deficiency may cause a congenital secondary erythrocytosis.
Consider screening those with:
- True erythrocytosis
- No polycythaemia vera
- No identifiable secondary cause
- Young patients
- If positive family history
Low erythropoietin (EPO) level —- consider lesions of EPO signalling pathway
Normal or raised EPO level —- consider gene mutations in the oxygen sensing pathway
20 mls peripheral blood in EDTA