Eosinophilia is defined as an elevated eosinophil concentration in the peripheral blood.
- Eosinophil counts of > 500/μl correspond to a weak eosinophilia.
- Eosinophil counts of 1500/μl correspond to moderate eosinophilia
- Eosinophil counts of > 5000/μl correspond to severe eosinophilia.
The causes are heterogeneous and require differentiation between a:
- primary eosinophilia due to a haematological neoplasm, e.g. eosinophil leukaemia (CEL)
- a secondary eosinophilia
- a reactive eosinophilia, e.g. caused by parasitic infestation or allergic reactions
- an ideopathic eosinophilia(hypereosinophilia syndrome - HES).
In the case of eosinophil leukemia (chronic eosinophil leukemia- CEL-NOS), the persistent eosinophilia is due to clonal proliferation of an eosinophil precursor.
Within the WHO major category, CEL-NOS is classified in the myeloproliferative neoplasia (MPN) group and can be diagnosed by an eosinophil concentration > 1500/μl, evidence of clonal cytogenetic or molecular genetic markers, and exclusion of the following criteria:
- Absence of a Philadelphia chromosome or a PDGFRA, PDGFRB, FGFR1, or JAK2 rearrangement.
- Exclusion of other acute or chronic myeloproliferative and/or myelodysplastic neoplasms.
- ≥ 20% blasts in peripheral blood or bone marrow.