This guideline discusses the differential diagnosis and investigation of patients presenting with eosinophilia, and then considers management options for patients with haematological causes of eosinophilia.

It focuses on:

1. Biology and epidemiology of eosinophilia

2. Secondary (reactive) eosinophilia and its causes

3. Clonal eosinophilia

4. Idiopathic hypereosinophilia and the idiopathic hypereosinophilic syndrome (HES)

5. Evaluation of patients presenting with eosinophilia: determining the underlying cause and assessing for eosinophil-mediated tissue damage

6. Emergency treatment

7. Treatment of primary (clonal) eosinophilic disorders

8. Treatment of lymphocyte variant HES

9. Treatment of idiopathic HES

Declaration of Interests

The BSH paid the expenses incurred during the writing of this guidance. None of the authors had conflicts of interest to declare. All authors have made a declaration of interests to the BSH and Task Force Chairs which may be viewed on request.