Eosinophils (EOS%): your blood test guide

What are eosinophils (EOS%)

Eosinophils are white blood cells belonging to the granulocyte group. They play an important role in fighting parasitic infections and participate in allergic reactions. Eosinophils can release substances that destroy parasites, but excessive activity can also damage the body's own tissues. EOS% shows the proportion of eosinophils among all white blood cells.

This marker is measured as part of a complete blood count with differential. The result is reported as a percentage.

Reference ranges

The eosinophil percentage in adults is normally 0 to 5 % of all leukocytes. Some laboratories report a range of 1–4 %. The result may vary slightly depending on the time of day (eosinophil counts tend to be lower in the morning, when cortisol levels are highest). Always compare your result with the reference range provided by your laboratory.

Why eosinophils may be elevated

An elevated eosinophil percentage (eosinophilia) is most commonly associated with allergic conditions — asthma, hay fever, atopic dermatitis, food allergy. Parasitic infections (particularly helminths) are a classic cause of eosinophilia, although they are less common in some regions.

Certain medications — antibiotics, non-steroidal anti-inflammatory drugs, anticonvulsants — can cause eosinophil elevation as a side effect. Autoimmune diseases such as eosinophilic granulomatosis with polyangiitis, and some haematological conditions, may also be associated with a persistently elevated count.

In cases of severe eosinophilia (more than 20 %), urgent medical consultation is necessary, as this may indicate a more serious condition.

Why eosinophils may be low

A decreased eosinophil percentage (eosinopenia) is usually a physiological phenomenon. Acute stress, surgical intervention, or glucocorticoid use (e.g. prednisolone) can temporarily reduce eosinophil counts. Acute bacterial infections also often reduce the eosinophil fraction, as the body directs neutrophils to the forefront.

Eosinopenia is generally not clinically significant if other markers are within normal limits.

What to do with your result

If eosinophils are elevated, the doctor will first look for allergic causes — asking about symptoms (cough, skin rash, nasal congestion) and ordering allergy tests or parasitology investigations. If the elevation is mild and there is a known allergy, managing it is usually sufficient.

A healthy diet, regular physical activity, and avoiding allergens help maintain normal immune balance.

Monitoring over time

Eosinophil counts can fluctuate seasonally — for example, they may rise in spring due to pollen allergy. By observing several tests, one can determine whether the elevation is temporary (seasonal) or persistent. The doctor finds a trend more useful than a single result.

Questions for your doctor

1. Could my eosinophil count be related to an allergy or another chronic condition? 2. Is it worthwhile to perform allergy tests or check for parasitic infections? 3. Could my current medications have affected the eosinophil count? 4. When should the test be repeated to monitor the trend?

*This is not medical advice. Consult your doctor regarding any health decisions.*