How I investigate Eosinophilia

Eosinophilia is typically secondary, that is, reactive, in nature and is associated with a wide variety of neoplastic and non-neoplastic disorders. Clonal eosinophilia is also seen in a wide variety of hematopoietic neoplasms, and sub-classification can be diagnostically challenging. A proper evaluation of persistent eosinophilia involves correlation of clinical history, laboratory data, cellular morphology, and ancillary testing. Knowledge of appropriate ancillary testing is necessary for a timely diagnosis. We present a review of the literature regarding eosinophilia, including the 2016 World Health Organization (WHO) update of WHO-defined eosinophilic disorders. We also present a review of eosinophilia in a case-based format including guidelines for evaluation of both routine and challenging cases. The purpose of this guideline is not to provide an in-depth discussion of each diagnosis, but rather a practical method that all pathologists can utilize to investigate eosinophilia.