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Eosinophil cationic protein (ECP)
(Marker of Eosinophil Activation – Allergy, Asthma, Atopy & Eosinophilic Inflammation)
Synonyms
- ECP
- Eosinophil cationic protein
- Ribonuclease 3 (RNase 3)
- Eosinophil granule protein
- Eosinophil-derived cytotoxic protein
Units of Measurement
- ng/mL
- ng/dL
- ng/100 mL
- ng%
- ng/L
- µg/L
1 ng/mL = 1 µg/L
1 ng/mL = 100 ng/dL
ng/100 mL = ng% = ng/dL
Description
Eosinophil Cationic Protein (ECP) is a toxic ribonuclease released from activated eosinophils.
It plays a major role in:
- Allergic inflammation
- Asthma airway hyperresponsiveness
- Tissue damage in eosinophil-mediated diseases
ECP measurement reflects eosinophil activation, not merely eosinophil count.
Therefore, ECP is a functional marker rather than a simple cell count indicator.
Physiological Role
ECP participates in:
- Antiparasitic activity
- Tissue remodeling
- Modulation of mast cells and T-cells
- Bronchial hyperreactivity
- Cytotoxicity in allergic airway inflammation
Released during:
- Degranulation
- Immune activation
- Allergic responses
Clinical Significance
Elevated ECP (Most Important)
1. Bronchial Asthma
- High ECP indicates airway inflammation
- Correlates with:
- Disease severity
- Nocturnal symptoms
- Airway hyperreactivity
- Exacerbations
- Disease severity
Used for:
- Monitoring therapy response
- Guiding inhaled corticosteroid titration
2. Allergic Diseases
Elevated in:
- Allergic rhinitis
- Atopic dermatitis
- Eosinophilic esophagitis
- Food allergies
- Drug allergies
3. Parasitic Infections
Strongly elevated due to eosinophil activation.
4. Hypereosinophilic Syndromes (HES)
Helps evaluate disease activity along with eosinophil counts.
5. Autoimmune & Inflammatory Disorders
Moderate elevation in:
- Eosinophilic granulomatosis with polyangiitis (EGPA)
- Eosinophilic pneumonia
Low / Normal ECP
- Normal eosinophil activation
- Well-controlled asthma
- Absence of significant allergic inflammation
Low ECP is not clinically concerning.
Reference Intervals
(Tietz 8E + EAACI + Mayo + ARUP)
Serum ECP
- < 24 ng/mL (most common cutoff)
Some labs use < 20–25 ng/mL.
Interpretation:
| ECP Level | Meaning |
| < 20–24 ng/mL | Normal |
| 25–50 ng/mL | Mild eosinophil activation |
| 50–100 ng/mL | Moderate activation |
| > 100 ng/mL | High activation (asthma flare, HES) |
In Atopic Dermatitis
Levels may exceed 200–300 ng/mL during flares.
In HES
May exceed 500 ng/mL.
Unit Meanings
| Unit | Meaning |
| ng/mL | nanograms per milliliter |
| ng/dL | nanograms per deciliter |
| ng/100 mL = ng% | identical |
| ng/L | nanograms per liter |
| µg/L | micrograms per liter |
Diagnostic Uses
1. Asthma Diagnosis & Monitoring
- Helps assess inflammatory phenotype
- Higher sensitivity for eosinophilic inflammation than blood eosinophil count
- Useful in corticosteroid titration
2. Atopic Dermatitis Severity
Correlates strongly with:
- Eczema Area and Severity Index (EASI)
- Itching intensity
- Flare frequency
3. Eosinophilic Gastrointestinal Diseases
Useful in:
- Eosinophilic esophagitis
- Eosinophilic gastroenteritis
4. Hypereosinophilic Syndrome
Tracks disease activity and treatment response.
5. Parasitic Infections
Elevated ECP supports diagnosis in eosinophilia associated with helminths.
Analytical Notes
- Serum preferred
- Sample should be processed quickly (ECP released during clotting)
- Collection tubes: avoid prolonged clotting
- Immunoassay-based (ELISA, fluoroimmunoassays)
- Diurnal variation minimal compared to eosinophil count
- Not useful in acute anaphylaxis (transient changes)
Clinical Pearls
- ECP reflects activation, not just number better for monitoring allergic inflammation.
- ECP is one of the best biomarkers for asthma control and steroid responsiveness.
- High ECP in atopic dermatitis correlates with barrier dysfunction.
- ECP can identify eosinophilic phenotype when blood eosinophils are normal.
- Rapid rise in ECP suggests upcoming asthma exacerbation even before symptoms.
Interesting Fact
ECP is highly cytotoxic - it can damage parasites, bacteria, epithelial cells, and even host tissues when excessively released in allergic diseases.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Immunology & Inflammatory Markers.
- EAACI Position Papers - Biomarkers in Asthma & Atopy.
- Mayo Clinic Laboratories - ECP.
- ARUP Consult - Eosinophilic Disorders.
- MedlinePlus / NIH — Allergy Laboratory Tests.