Eosinophils in peripheral blood account for 0.3-5% of leukocytes, which is equivalent to 0.05-0.5 x 109/l. A count equal or above 0.5 x 109/l is considered eosinophilia, while a count equal or above 1.5 x 109/l is defined as hypereosinophilia. In bone marrow, eosinophilia is considered when eosinophils make up more than 6% of the total nuclear cells. In daily clinical practice, the most common causes of reactive eosinophilia are non- hematologic, whether non-neoplastic (allergic diseases, drugs, infections or immunological diseases) or neoplastic (solid tumors). Eosinophilia associated with a haematological malignancy may be reactive or secondary to the production of eosinophilopoietic cytokines, and this is mainly seen in lymphoid neoplasms (Hodgkin lymphoma, mature T-cell neoplasms, lymphoid variant of hypereosinophilic syndrome and B-acute lymphoblastic leukemia/lymphoma). Eosinophilia associated with a haematological malignancy may also be neoplastic or primary, derived from the malignant clone, usually in myeloid neoplasms or with origin in stem cell (myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions, acute myeloid leukemia with Core Binding Factor translocations, mastocytosis, myeloproliferative neoplasms, myelodysplastic/myeloproliferative neoplasms and myelodysplastic neoplasms). There are no concrete data in standardized cytological and cytometric procedures that could predict whether eosinophilia is reactive o clonal. The verification is usually indirect, based on the categorization of the accompanying hematologic malignancy. This review focuses on the broad differential diagnosis of haematological malignancies with eosinophilia.

There is remarkable morphologic and genetic heterogeneity in acute myeloid leukemia (AML). In a small percentage of cases of AML, increased eosinophils and/or basophils are present in the bone marrow and sometimes in the peripheral blood. This is often a puzzling diagnostic situation but also an important finding that requires special investigation. Unique chromosomal rearrangements have been correlated with an increased number of eosinophils and basophils in AML. Identification of the underlying genetic lesion that promotes eosinophilia and basophilia can dramatically change both the prognosis and the treatment of the patient. Thus clinicians must be vigilant in searching for the cause of eosinophilia and/or basophilia in patients with AML, since the different causes may lead to different treatments and survival outcomes. In this article we examine the significance of increased eosinophils and/or basophils in the context of AML, provide guidance that simplifies the differential diagnosis, and prognostic and therapeutic information about specific subtypes of AML associated with eosinophilia and/or basophilia. Evidence supporting personalized (molecularly targeted) therapy for these patients is also presented.

Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) is a rare and potentially life-threatening systemic drug reaction with skin involvement. We present the unusual case of DRESS in a 16-year-old male that was treated with TMP-SMX for acne and was initially misdiagnosed with Steven Johnson Syndrome. Our case serves as an example to healthcare providers treating adverse drug reactions to having a high clinical suspicion for DRESS as delay in diagnosis and treatment can result in disseminated disease and higher patient mortality risk.