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]
Summary
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, and several different classifications of eosinophilic disorders have been proposed. Peripheral blood eosinophilia has traditionally been divided into mild (500-1499/microliter), moderate or marked (1500-5000/microliter), and severe (>5000/microliter). Consensus recommendations define persistent hypereosinophilia as >1500/microliter recorded on at least 2 occasions ≥2 weeks apart.[2] 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.[2]
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.
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Blood film from a patient with reactive eosinophilia occurring as a response to acute lymphoblastic leukemia. There is 1 blast cell. Eosinophils are cytologically abnormal
Blood film from a patient with chronic eosinophilic leukemia resulting from an FIP1L1-PDGFRA fusion gene. The eosinophils are partly degranulated and 1 is hypolobated
Citations
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Butt NM, Lambert J, Ali S, et al; British Society for Haematology. Guideline for the investigation and management of eosinophilia. Br J Haematol. 2017 Feb;176(4):553-72.[Abstract][Full Text]
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Shomali W, Gotlib J. World Health Organization-defined eosinophilic disorders: 2022 update on diagnosis, risk stratification, and management. Am J Hematol. 2022 Jan 1;97(1):129-48.[Abstract][Full Text]
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