Eosinophils: what they are, what they tell you and why we test them

Eosinophils are the white blood cells involved in allergy and in fighting parasites. There are very few of them in the blood, and your report may even count zero. They rise mostly with allergies, asthma or eczema, sometimes with a medicine or after travel.

Why we test them

Eosinophils are one of the five types of white blood cell in a full blood count, in the part called the differential. They fight parasites and take part in allergies: they gather where there is a reaction, for example in the airways with asthma.

There are very few in the blood: most live in the tissues, such as the gut. When they rise in the blood, an allergy is often behind it, though it is not the only cause.

A low value, on the other hand, usually means nothing.

What they tell you, and what they do not

If they are high, an allergy is often behind it: hay fever, asthma, eczema. Other times a medicine or, after travel, a parasite. A cause is not always found.

If they are low or zero, that is usually normal: there are so few in the blood that it happens often.

And cortisone lowers them within hours. A normal result while you take it does not rule out an allergy.

That is why they are read together with other tests:

Normal eosinophil levels

Your report usually lists eosinophils twice. The percentage says what share of your white cells are eosinophils. The absolute count says how many there are in a microlitre of blood. If there is only the percentage, the absolute count is the percentage times your white cell count, divided by a hundred.

The lab decides what is normal, and prints its limits next to each line. On some reports the lower limit is zero: a result of zero is common and normal.

The absolute count is written as x10⁹/L or x10³/µL: it is the same number. Some reports give it per mm³, multiplied by a thousand. The absolute count is what shows whether they are really high.

In the Rosa report you see your eosinophils next to the rest of your blood count. Rosa also calculates the absolute count, from your total white cells.

If your eosinophils are high or low

What can skew the result

A few things change the number for a while. It helps to know them before you read it.

When to worry

Talk to your doctor if:

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

What are eosinophils? White blood cells that fight parasites and take part in allergies. There are very few in the blood: most live in the tissues, such as the gut.

What are normal eosinophil levels? The lab decides, and your report prints its limits for the percentage and the absolute count. Zero is usually normal; above the limit they count as high, often because of an allergy. At 1.5 x10⁹/L or more, talk to your doctor.

Do high eosinophils mean an allergy? Often, yes: allergies, asthma and eczema are among the most common causes. But they are not the only ones, and sometimes no cause is found. If they stay high, your doctor decides whether more tests are needed.

How often should they be checked? They are part of the blood count: Rosa measures them twice a year. If they are out of range, your doctor decides whether and when to repeat them.

Sources