Evaluation of eosinophilia

Eosinophilia is defined as an increase in the peripheral blood eosinophil count. The upper limit of normal is usually taken as about 600/microliter, but it is lower (about 400/microliter) if people with minor allergies are excluded. There is no ethnic variation in the normal eosinophil count and no physiologic cause of an increased count.[1]Bain BJ. Blood cells: a practical guide. 5th ed. Oxford: Wiley-Blackwell; 2015.

Causes of eosinophilia

The most common cause of eosinophilia depends on socioeconomic and geographic 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.

Hypereosinophilia

The terms eosinophilia and hypereosinophilia do not have precise definitions. 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

Follows the traditional model of clinical history, physical exam, and laboratory and other investigations, taking into account the likely causes in the individual patient.