High eosinophils: what it means and what to do
High eosinophils are common. An allergy is often behind them, such as hay fever, asthma or eczema, other times a medicine or, after travel, a parasite. Look at the absolute count, and whether it stays high over time.
What high eosinophils mean
This is called eosinophilia. Your body makes more eosinophils than normal, often because of an allergic reaction or to fight off a parasite.
Look at the absolute count, not the percentage. A result just above your report’s limit is common and often explained by an allergy. At 1.5 x10⁹/L or more, or if it stays high over time, it should be looked into with your doctor.
What causes high eosinophils
The first ones are the most common. Sometimes no cause is found: if the value stays stable, your doctor keeps an eye on it over time.
- Allergies: Hay fever, allergy to dust mites or pet hair. They rise more in the pollen season.
- Asthma and eczema: They often come with allergies, and keep eosinophils a little higher.
- Some medicines: Some antibiotics, anti-inflammatories or epilepsy medicines can raise them. Tell your doctor, and do not stop them on your own.
- Parasites: Especially after a trip to a tropical country, but not only. They often cause no symptoms.
- Autoimmune diseases: Some inflammatory and autoimmune diseases can raise them.
- More rarely, other causes: If they are very high and stay high, your doctor also looks for rarer causes, such as some blood diseases.
Symptoms of high eosinophils
High eosinophils on their own often cause no symptoms. If there are any, they come from the cause: a blocked nose and sneezing, itching, dry red skin, a cough or breathlessness with asthma.
A parasite can bring diarrhoea, belly pain, itching or weight loss, but often nothing at all. A cough or breathlessness that will not go away should be seen by a doctor.
High eosinophils after travel
If you have travelled to or lived in a tropical country, even months or years ago, tell your doctor. In people back from a trip, high eosinophils often come from a parasite, even without symptoms.
It is usually looked for with blood and stool tests, and treated with targeted medicines. It is worth finding, because some parasites can stay in the body for years, and cortisone can make the infection worse.
Which tests to read alongside them
These tests help show where it comes from.
What to do if they are high
It depends on the cause. Where to start:
- Look at the absolute count, not just the percentage.
- Think about what has changed: seasonal allergies, a new medicine, a trip.
- Show the report to your doctor, who decides whether to repeat the test and whether other checks are needed, for example for allergies or parasites.
- If you started a new medicine recently, tell your doctor and do not stop it on your own. If you also have a rash and a fever, see a doctor the same day.
- If you have an allergy or asthma, keeping it under control with your doctor usually brings them down.
When to worry
Talk to your doctor if:
- Your absolute eosinophil count is 1.5 x10⁹/L or more.
- They stay above your report’s limit on a second test, without a known allergy.
- They are high and you have travelled to or lived in a tropical country, even if you feel well.
- They are high and you have a cough or breathlessness that will not go away, diarrhoea, belly pain or weight loss.
- In the weeks after starting a new medicine you get a widespread rash and a fever: see a doctor the same day.
- With the rash you get blisters, peeling skin or sores in your mouth or eyes: call 112 or go to the emergency department.
- You get suddenly short of breath or have chest pain: call 112 or go to the emergency department.
With Rosa, eosinophils of 1.5 x10⁹/L or more (absolute count) are always flagged to a doctor, who usually has the test repeated.
Frequently asked questions
High eosinophils: when should I worry? Usually you do not need to: an allergy is often behind them. Talk to your doctor if the absolute count is 1.5 x10⁹/L or more. Do the same if they stay high on a second test without a known allergy, or if you have travelled to or lived in a tropical country.
Which tests are needed if they stay high? Your doctor decides, based on your history and symptoms. They usually repeat the blood count and may ask for allergy tests, or blood and stool tests for parasites.
How long do they take to come down? It depends on the cause. After the pollen season, or once your doctor stops the medicine that raised them, they usually come down within a few weeks. With a parasite, after treatment.
Can high eosinophils mean cancer? Rarely. The most common causes are allergies, asthma, eczema, medicines and parasites. If they are very high and stay high, your doctor also looks for the rarer causes.
Sources
- Valent and colleagues, refined criteria to define and classify eosinophil disorders. Allergy, 2023. PMID 36207764
- Patel and colleagues, the causes of high eosinophils in people referred to a haematologist, and the level at which to refer. European Journal of Haematology, 2026. PMID 41833286
- Hong and colleagues, why cortisone lowers eosinophils within hours. Blood, 2020. PMID 32659786
- Thakker and colleagues, investigating high eosinophils in returning travellers and migrants. Journal of Infection, 2025. PMID 39537036
