Diagnosis is based on the exclusion of reactive causes (i.e., parasitic infection; drug reaction; allergy and collagen vascular disorders) or clonal causes (i.e., chronic eosinophilic leukemia, not otherwise specified; myeloid-lymphoid neoplasms with eosinophilia associated with rearrangements of PDGFRA, PDGFRB, FGFR1, or PCM1-JAK2 [1–4] and lymphocyte-variant hypereosinophilia) for eosinophilia. Here, FGFR1 is linked to Increased total eosinophil count.