Eosinophils in a blood test: normal range and results (EOS)
Less than 500 cells/µl (converted); laboratory intervals can differ slightly
- Updated
- 2026-09-10
- Every number with its source
What eosinophils are
Eosinophils are a type of white blood cell (leukocyte) made in the bone marrow. They make up a small share of the whole white blood cell count, but they have a distinct function — they take part in fighting parasites (especially helminths) and in regulating the immune response in allergic reactions such as hay fever, asthma or eczema. The amount of eosinophils is determined by a full blood count with a white cell differential, or by a separate absolute eosinophil count (a blood differential). The result can be given as an absolute number (cells per microlitre or ×10⁹/l) or as a percentage of the whole white cell count. A raised eosinophil count (eosinophilia) most often indicates allergy or a parasitic infection, but it can also be linked to autoimmune diseases, some skin diseases or, in rare cases, diseases of the blood. A reduced amount is clinically significant less often and is most often linked to the effect of steroids on the body or to alcohol intoxication.
The reference range for eosinophils
| Group | Range, ×10⁹/l | Source |
|---|---|---|
| Adults and children Less than 500 cells/µl (converted); laboratory intervals can differ slightly | less than 0.5 | MedlinePlus Reviewed 2025-01-28 |
When eosinophils are tested
- As part of a full blood count with a white cell differential — for prevention or while ill
- When an allergic reaction is suspected — hay fever, asthma, eczema, urticaria (hives)
- When a parasitic infection is suspected
- When assessing inflammation of unclear origin or an autoimmune disease
- When monitoring hypereosinophilic syndrome or diseases of the blood
- When a deviation has been confirmed in a full blood count (in the white cell differential)
How to prepare
- For adults, special preparation is usually not needed
- Tell your doctor about the medicines you take — some preparations (for example amphetamines, certain antibiotics, laxatives, interferon, sedatives) can temporarily raise the eosinophil count
A raised eosinophil count (eosinophilia)
A raised eosinophil count (eosinophilia) is most often linked to allergic diseases — hay fever, asthma, eczema — or to a parasitic infection. Other causes are adrenal insufficiency, autoimmune diseases, fungal infections, hypereosinophilic syndrome, leukaemia and other diseases of the blood, and lymphoma. Other tests and the clinical picture help to tell the cause apart.
- Allergic diseases — hay fever, asthma, eczema
- A parasitic infection
- Adrenal gland insufficiency
- Autoimmune diseases
- Fungal infections
- Hypereosinophilic syndrome
- Leukaemia and other diseases of the blood
- Lymphoma
What is usually assessed alongside a raised result:
- Whether there are signs of allergy or exposure to a known allergen
- Travel history and a possible parasitic infection
- A repeat full blood count with the complete white cell differential
- When to repeat the test so that the change over time can be seen
A reduced eosinophil count
A reduced eosinophil count is clinically significant less often than a raised one. According to MedlinePlus, it can be linked to alcohol intoxication, or to increased production of steroid hormones (for example cortisol) in the body, or to taking steroid medicines.
- Alcohol intoxication
- Increased production of steroid hormones (cortisol) in the body
- Taking steroid medicines
What is usually assessed alongside a reduced result:
- Whether steroid medicines were being taken at the time of the test
- A full blood count with the complete white cell differential together with the other values
- When to repeat the test to assess the change
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Frequently asked questions
›What is the normal eosinophil count in blood?
According to the MedlinePlus medical encyclopedia, a normal eosinophil count is less than 500 cells per microlitre, that is about 0.5 ×10⁹/l. Laboratory intervals can differ slightly — you will find the exact interval on your own report.
›Why can eosinophils be raised?
The most common causes are allergic diseases (hay fever, asthma, eczema) and a parasitic infection. Less often a raised count is caused by adrenal insufficiency, autoimmune diseases, fungal infections, hypereosinophilic syndrome, or diseases of the blood such as leukaemia or lymphoma.
›Do raised eosinophils always mean allergy?
Not always. Although allergy is the most common cause, raised eosinophils can also indicate a parasitic infection, an autoimmune disease or rarer diseases of the blood. The exact cause is established by a doctor, assessing the result together with the other tests and the symptoms.
›Why can eosinophils be reduced?
A reduced eosinophil count is clinically significant less often than a raised one. It can be linked to alcohol intoxication, or to increased production of steroid hormones such as cortisol in the body, and also to taking steroid medicines.
›Is any special preparation needed before an eosinophil test?
For adults, special preparation is usually not needed. It is worth telling your doctor about the medicines you take, because some preparations can temporarily raise the eosinophil count in the blood.
›How are eosinophils measured — on their own or together with other tests?
The eosinophil count is most often determined as part of a full blood count with a white cell differential, but a doctor can also order a separate absolute eosinophil count if allergy or a parasitic infection is suspected, or to clarify the result of a full blood count.
Sources
Related markers
This site is not an emergency service. If severe chest or head pain, breathlessness, confusion, difficulty speaking, heavy bleeding or another acute problem has come on suddenly — do not wait for tests, call 112.
This information is not a diagnosis and does not replace a consultation with a doctor. Reference ranges depend on the laboratory, your age and sex — always compare your result with the range printed on your own report.