The recent discovery of an eosinophilia-specific, imatinib-sensitive, karyotypically occult but fluorescence in situ hybridization-apparent molecular lesion in a subset of patients with blood eosinophilia has transformed the diagnostic as well as treatment approach to eosinophilic disorders. Primary (i.e. nonreactive) eosinophilia is considered either ‘clonal’ or ‘idiopathic’ based on the presence or absence, respectively, of either a molecular or bone marrow histological evidence for a myeloid neoplasm. Clonal eosinophilia might accompany a spectrum of clinicopathological entities, the minority of whom are molecularly characterized; Fip1-like-1-platelet-derived growth factor receptor α (FIP1L1-PDGFRA+) systemic mastocytosis, platelet-derived growth factor receptor β (PDGFRB)-rearranged atypical myeloproliferative disorder, chronic myeloid leukemia, and the 8p11 syndrome that is associated with fibroblast growth factor receptor 1 (FGFR1) rearrangement. Hypereosinophilic syndrome (HES) is a subcategory of idiopathic eosinophilia and is characterized by an absolute eosinophil count of ≧1.5 × 109/l for at least 6 months as well as eosinophil-mediated tissue damage. At present, a working diagnosis of primary eosinophilia mandates a bone marrow examination, karyotype analysis, and additional molecular studies in order to provide the patient with accurate prognostic information as well as select appropriate therapy. For example, the presence of either PDGFRA or PDGFRB mutations warrants the use of imatinib in clonal eosinophilia. In HES, prednisone, hydroxyurea, and interferon-α constitute first-line therapy, whereas imatinib, cladribine, and monoclonal antibodies to either interleukin-5 (mepolizumab) or CD52 (alemtuzumab) are considered investigational. Allogeneic transplantation offers a viable treatment option for drug-refractory cases.

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