Background
An eosinophil count measures the number of eosinophils in peripheral blood. Eosinophils, capable of phagocytosis, ingest antigen-antibody complexes and become active in the later stages of inflammation. Eosinophils respond to allergic and parasitic diseases. Eosinophilic granules contain histamine and account for one-third of all the histamine in the body. Eosinophilia occurs when there is an increased number of circulating eosinophils; conversely, eosinopenia occurs when there is a decreased number of circulating eosinophils. This test is used to diagnose allergic disorders, assess severity of parasitic infections, and monitor response to treatment.
Normal reference values in adults
Absolute eosinophil count (determined by multiplying the total white blood cell (WBC) count by the percentage of eosinophils): 0 to 500 cells/mcL (<0.5 x 109/L)
Differential: 0%-3% of total WBC count
Possible causes of eosinophilia
(>5% or >0.5 x 109/L)
* allergies, asthma
* parasitic disease
* some endocrine disorders, such as Addison disease, hypopituitarism
* Hodgkin disease and myeloproliferative disorders, chronic myeloid leukemia, polycythemia vera
* chronic skin diseases, such as eczema
* some infections, such as HIV
* idiopathic hypereosinophilic syndrome and autoimmune and idiopathic disorders, such as sarcoidosis, inflammatory bowel disease, immunoglobulin G4-related disease, and other connective tissue disorders and immunodeficiency disorders
* aspirin sensitivity, allergic drug reactions
* poisons, such as phosphorus
Possible causes of eosinopenia
(<1% or 0.0 cells/mm3 or 0.0 x 109/L)
Eosinopenia is usually caused by an increased adrenal steroid production that accompanies most stress disorders and is associated with:
* Cushing syndrome
* use of certain drugs, such as epinephrine, thyroxin
* acute bacterial infections
Interfering factors
* Normal eosinophil count is lowest in the morning, then rises from noon until after midnight. For this reason, serial eosinophil counts should be repeated at the same time each day.
* Stressful situations, such as burns, trauma, and postoperative states, cause a decreased eosinophil count.
* After administration of corticosteroids, eosinophils disappear.
Sources: Fischbach FT, Fischbach MA. A Manual of Laboratory and Diagnostic Tests. 10th ed. Philadelphia, PA: Wolters Kluwer; 2018. Wei S, Kao LS. Eosinopenia and adverse outcomes after Clostridium difficile infections: of mice and men. JAMA Surg. 2018;153(12):1133-1134. Matono T, Kutsuna S, Kato Y. Role of classic signs as diagnostic predictors for enteric fever among returned travellers: relative bradycardia and eosinopenia. PLoS ONE. 2017;12(6):e0179814.