Approach to Patients with Eosinophilia.

Physicians may encounter blood or tissue eosinophilia through a routine complete blood count with differential or a tissue pathology report. In this article, the basic biology of eosinophils is reviewed and definitions of blood eosinophilia, as well as the challenges of defining tissue eosinophilia, are discussed. Conditions associated with eosinophilia are briefly discussed as well as a general approach to evaluating eosinophilia. Future challenges include determining which eosinophil-associated diseases benefit from eosinophil-targeted therapy and identifying biomarkers for disease activity and diagnosis.