Eosinophilia is defined as an increase in the peripheral blood eosinophil count. The upper limit of normal is usually taken as about 0.6 x 10^9/L (600/microlitre), but it is lower (about 0.4 x 10^9/L [400/microlitre]) if people with minor allergies are excluded. There is no ethnic variation in the normal eosinophil count and no physiological cause of an increased count. 
Causes of eosinophilia
The most common causes of eosinophilia depend on socioeconomic and geographical factors. In the developing world the most common cause is parasitic infection (and parasitic infections result in the greatest number of cases worldwide), whereas in the developed world it is allergy. Uncommon and rare causes are important because eosinophilia may be the result of a serious underlying condition.
The terms eosinophilia and hypereosinophilia do not have any precise definition. Nevertheless, the higher the eosinophil count, the more urgent it is to determine the cause, first because the underlying disease may need treatment in its own right, and second because hypereosinophilia can lead to tissue damage, which can be life-threatening. If eosinophils are extensively degranulated, tissue damage is more likely because of the enzymes, cytotoxic proteins, and cytokines released.
Investigation of eosinophilia
Investigation of eosinophilia should follow the traditional model of clinical history, physical exam, and laboratory and other investigations, taking into account the likely causes in the individual patient.
- Ancylostoma caninum infestation
- Herpes gestationalis
- Drug hypersensitivity
- Allergic bronchopulmonary aspergillosis
- Churg-Strauss syndrome
- Chronic eosinophilic leukaemia
- Other leukaemias
- Non-Hodgkin's lymphoma
- Hodgkin's lymphoma
- Non-haematological malignancy-associated hypereosinophilia
- Idiopathic hypereosinophilic syndrome