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The eosinophil count in human blood is normally 0.4 × 109/L (0.1 - 0.6) and results from a balance between production of eosinophils and emigration through post-capillary venules (Yamaguchi et al. 1991). Eosinophils are only a small minority of peripheral blood leucocytes and in normal subjects, most are found in the tissues of the lung and gastro-intestinal tract (Beeken et al. 1987). Blood eosinophil counts are arbitrarily classified as mild - between 0.6 to 1.5 × 109/L; moderate between 1.5 to 5 × 109/L and severe when greater than 5 × 109/L.
An elevated blood eosinophil count may be associated with a number of reactive conditions and with clonal disorders of the bone marrow. However, when the blood eosinophil count is persistently greater than 1.5 × 109 /L, for a period of more than six months, damage to end organs such as the heart, lungs, skin, joints and nervous system can be demonstrated, and in the absence of any clonal or reactive cause, the term idiopathic hypereosinophilic syndrome (HES) is used. The three defining criteria of HES are therefore:
- Eosinophil count persistently greater than 1.5 × 109/L
- Damage to end-organs
- No ascertainable cause for the eosinophilia and no evidence of clonality.
There are three categories of blood eosinophilia:
- Reactive (non-clonal) eosinophilia: infections, parasitic infestations, asthma and allergies, respiratory diseases, cytokine infusions, vasculitides, non-haematological malignant diseases, drug reactions and connective tissue diseases, Hodgkin's and non-Hodgkin's lymphomas are included here as the eosinophils have not been shown to be clonal.
- Clonal Disorders of the Bone Marrow associated with eosinophilia: Acute and chronic eosinophilic leukaemia, chronic myeloid leukaemia, polycythaemia rubra vera, essential thrombocythaemia, acute myeloid leukaemia. Chromosome 16 variants, the 8p11 myeloproliferative syndrome (EMS) and T lymphoblastic lymphoma with eosinophilia, acute lymphoblastic leukaemia, myelodysplastic disorders (MDS) with eosinophilia, systemic mastocytosis and acute lymphoblastic leukaemia (Bain 1996).
- HES: After exclusion of the above two categories, cases of persistent, unexplained eosinophilia fall into the category of HES.