Low eosinophils: what it means and what to do

Low eosinophils, even at zero, usually mean nothing. There are very few in the blood, and a low result is common in people who feel well. Cortisone, severe stress or an infection also lower them.

What low eosinophils mean

This is called eosinopenia. Eosinophils are among the rarest white cells, and on some reports the lower limit is zero.

That is why a low result on its own is usually not a problem and needs nothing.

What causes low eosinophils

Usually the cause is temporary, or there is none at all.

• Nothing in particular: In many healthy people there are simply very few, even zero.
• Cortisone: As tablets or injections it lowers them within hours. They come back up after the treatment ends.
• Severe stress: Severe physical stress, like an operation or a serious illness, lowers them: the body makes more cortisol.
• An acute infection: During an infection they often drop, and rise again as you recover.
• Time of day: They are lower in the morning than in the evening: that is their normal rhythm.

Symptoms of low eosinophils

None. Low eosinophils cause no symptoms on their own.

If you feel unwell, it usually comes from the cause, such as an infection, and not from the eosinophils.

Zero eosinophils: is it normal?

Yes, usually. There are so few eosinophils that the count can reach zero even in people who feel perfectly well.

The rest of your blood count matters more. If your white cells, neutrophils and lymphocytes are normal, zero eosinophils on their own are nothing to worry about.

Which tests to read alongside them

These tests show whether there is something to look into.

What to do if they are low

Usually nothing. Where to start:

• If you feel well and the rest of your blood count is normal, you do not need to do anything.
• If you take cortisone, low eosinophils are expected: do not stop it on your own.
• If they are low together with other white cells, show the report to your doctor. If you also have a fever, see a doctor the same day.

When to worry

Usually you do not need to. Talk to your doctor if:

• They are low together with other blood count values out of range.
• You have had a fever or felt unwell for days.
• They are low together with other white cells and you have a fever: see a doctor the same day.
• You have a fever with very low neutrophils: call 112 or go to the emergency department.
• You have a high fever and feel confused, or you get suddenly short of breath: call 112 or go to the emergency department.

Frequently asked questions

Do low eosinophils mean weak immunity? No. Eosinophils mainly fight parasites and take part in allergies. A low result on its own does not show that your defences are weak.

Does cortisone lower eosinophils? Yes, within hours, and they usually stay low while you take it. That is why a low result during treatment is expected.

Low eosinophils and high neutrophils: what does it mean? Often it is the body’s response to an infection or severe stress, or the effect of cortisone. It usually settles once the cause passes. If you have a fever or feel unwell, talk to your doctor.

How often should they be checked again? If you feel well, with your usual checks: Rosa measures them twice a year.

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