Eosinophilia is defined as an abnormally high level of eosinophils in the blood, this is usually defined as >500 cells/μL (normal eosinophil level: <450 cells/μL). Hypereosinophilia is defined as >1500 cells/μL and is usually due to hematological malignancy .

This article includes recommendations and material from the "Guideline for the investigation and management of eosinophilia" published by the British Society for Hematology in 2017 .


The number of eosinophils is increased in neonates, cf. adults, with a gradual tailing off of blood counts in advanced age; no gender/ethnic variation is known .

The known causes of elevation of the eosinophil count is very broad, and usefully the etiology can be subdivided into secondary, primary and idiopathic forms. Secondary (reactive) type is by far the commonest.


A large number of primary hematological neoplastic conditions are associated with a clonal expansion of eosinophils.

  • myeloid and lymphoid neoplasms

    • chronic eosinophilic leukemia, not otherwise specified (CEL, NOS)

    • atypical chronic myeloid leukemia with eosinophilia (aCML-Eo)

    • chronic myelomonocytic leukemia with eosinophilia (CMML-Eo)

    • chronic myelogenous leukemia in accelerated phase or transformation

    • other myeloproliferative neoplasm in transformation

    • acute myeloid leukemia with eosinophilia (AML-Eo)

      • especially t(8;21)(q22;q22.1) or inv(16)(p13.1q22)

    • acute lymphoblastic leukemia, only if eosinophils demonstrated to be part of the neoplastic clone

    • systemic mastocytosis
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