Eosinophils (EOS#): Guide
Learn what eosinophils are, normal ranges (0.0–0.5 × 10³/µL), why they may be high or low, and what to do.
Updated: 9/21/2026
Reference ranges (typical adult values)
| Who | Range | Unit |
|---|---|---|
| Adults | 0 – 0.5 | 10³/µL |
Ranges differ between laboratories — the range printed on your own report always applies first.
What Are Eosinophils
Eosinophils are a type of white blood cell belonging to the granulocyte group. They are recognized by their brightly staining granules and play an important role in fighting parasitic infections and regulating allergic responses. Eosinophils produce enzymes and cytotoxic proteins capable of destroying parasites, but they also participate in tissue inflammation. The absolute eosinophil count (EOS#) more accurately reflects the eosinophil level than the percentage, as it is independent of fluctuations in total white blood cell count.
Reference Ranges
For adults, the absolute eosinophil count typically ranges from 0.0 to 0.5 × 10³/µL (or 0.0–0.5 × 10⁹/L in SI units). Some laboratories may set a narrower upper limit of 0.4 × 10³/µL. Always follow the reference ranges provided by your own laboratory.
Why Eosinophils May Be High
Elevated eosinophil count (eosinophilia) is most commonly associated with allergic conditions: asthma, allergic rhinitis, eczema, and food allergies. Parasitic infections — especially helminths — are a classic cause of eosinophilia, particularly relevant for travelers from tropical regions. Drug reactions, especially to antibiotics and antiepileptic medications, can cause elevation. Less commonly, eosinophilia may be associated with autoimmune diseases, eosinophilic esophagitis, or hematological conditions.
Why Eosinophils May Be Low
Very low or zero eosinophil count (eosinopenia) is most commonly seen during acute stress or infection — the effect of glucocorticoids (both endogenous stress and medications) is the main cause, as these hormones promote eosinophil apoptosis and migration from the blood. Cushing syndrome can result in persistently low eosinophil levels. After surgery or trauma, eosinophils also commonly decrease temporarily.
What to Do Next
Mild eosinophilia (0.5–1.5 × 10³/µL) is often related to allergic conditions and usually does not require urgent investigation, but it is worth discussing with your doctor, especially if symptoms are present. Moderate and severe eosinophilia requires more thorough evaluation: allergy testing, parasitological tests, and organ damage assessment. If eosinophils are low, this is usually a temporary phenomenon.
Tracking Over Time
Eosinophil level trends help distinguish temporary changes from chronic eosinophilia. Seasonal increases may indicate an allergic component. If eosinophils return to normal range after treatment, this confirms that the cause was successfully addressed. In the QbiLabs timeline, you can see trends and compare with CRP and other markers.
Questions for Your Doctor
- Could my elevated eosinophils be related to allergies or a seasonal reaction?
- Should parasitological tests be performed?
- Could my medications be affecting my eosinophil levels?
- How often should I repeat the test to monitor eosinophil trends?
*This is not medical advice. Discuss health decisions with your doctor.*