Ján Lazúr
Eosinophilia is a common haematological abnormality with different aethology. The number of eosinophils higher than 1,5 x 109/l is called hypereosinophilia, and it often causes tissue damage. Eosinophilias can be divided into secondary, clonal and idiopathic. The secondary forms are usually caused by parasites in the developing countries. On the other hand, in the developed countries the major cause is drugs and allergies. After excluding of secondary eosinophilia it is necessary to look for a clonal cause. According to WHO classification 2008, there are two main categories of clonal eosinophilias: 1. chronic eosinophilic leukaemia- not otherwise specified, 2. myeloid/lymphoid neoplasia with eosinophilia and mutations of platelet-derived growth factor receptor α/β (PDGFR A/B) or fibroblast growth factor receptor 1 (FGFR1). Then, after ruling out a secondary and clonal form, idiopathic eosinophilia is considered. If idiopathic eosinophilia has clinical manifestation, it is called idiopathic hypereosinophilic syndrome.