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Eosinophils serve many functions in the body, including mediation of allergic responses, eradication of parasitic infections, and down-regulation of the inflammatory response. Eosinophilia is defined as the presence of an absolute eosinophil count of more than 500 per microliter (uL) of blood. Absolute eosinophil count refers to the number of circulating eosinophils in the peripheral blood expressed as cells per uL. Eosinophilia can be classified as mild (500-1500 eosinophils/uL), moderate (1500-5000 eosinophils/uL), or severe (>5000 eosinophils/uL).

Two terms commonly used to describe eosinophilia include:

  • Hypereosinophilia, defined as moderate to severe eosinophilia (>1500 eosinophils/μL) with or without end-organ damage, and
  • Hypereosinophilic syndrome (HES), defined as hypereosinophilia demonstrated on at least two different occasions along with associated end organ damage.

Absolute eosinophilia occurs in benign conditions such as allergic reactions and infectious diseases as well as neoplastic disorders such as myeloproliferative and lymphoproliferative diseases. Exposure history including occupation, recreational activities, medications, supplements and food consumption is an important aspect of the initial evaluation. Examples include a risk of; strongyloides infection in miners, ascariasis in slaughterhouse workers, and schistosomiasis in river rafters.

Drug reactions to penicillin, cephalosporin, NSAIDs, proton pump inhibitors, allopurinol, phenytoin, aspirin, hydrochlorthiazide, carbamazepine and statins may be associated with eosinophilia. Manifestations of drug-induced eosinophilia can range from an asymptomatic eosinophilia to clinically significant end-organ involvement, such as the drug reaction with eosinophilia and systemic symptoms syndrome (DRESS). Symptoms of DRESS syndrome often develop 2 to 8 weeks after initiation of a new drug.

The following laboratory tests should be included in the initial workup:

  • CBC with differential and peripheral blood smear review to detect abnormalities in other cell lines and dysplasia
  • Serum creatinine and urinalysis for renal insufficiency
  • Serum vitamin B12 level which is often elevated in myeloproliferative disorders
  • Serologic testing for parasites especially for immigrants, patients with a history of travel or consumption of raw or incompletely cooked food. Possibilities include Strongyloides, Toxocara, Trichinella, Schistosomiasis, Ancylostomiasis, Ascariasis and Filariasis
  • Stool studies for ova and parasites
  • Liver function tests to assess hepatic involvement
  • Troponin levels to assess cardiac involvement
  • Flow cytometric analysis for lymphocyte subsets to detect lymphoproliferative disorders

The primary goal of an initial evaluation is to identify disorders requiring specific treatments such as parasitic infection, drug-hypersensitivity or leukemia.

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