Eosinophilia Definition and Classification

Eosinophilia is defined as a peripheral blood eosinophil count greater than 0.45 × 10^9/L (450 cells/μL). 1

Classification of Eosinophilia

Eosinophilia can be classified based on severity:
- Mild: 500-1500 cells/μL 2
- Moderate: 1500-5000 cells/μL 2
- Severe: >5000 cells/μL 2, 3
Hypereosinophilia (HE) is defined as a persistent elevated eosinophil count >1.5 × 10^9/L in blood on two examinations at least one month apart and/or tissue hypereosinophilia 1, 4

Hypereosinophilia is classified into four main variant types:

Hereditary (familial) HE: Characterized by familial clustering without signs of hereditary immunodeficiency 4
Primary (clonal/neoplastic) HE: Characterized by neoplastic proliferation of eosinophils, associated with WHO-defined myeloid and/or lymphoid neoplasms 4
Secondary (reactive) HE: Caused by an underlying condition where eosinophils are non-clonal and usually cytokine-driven (particularly IL-5) 4
HE of undetermined significance (HE-US): No underlying cause identified and no family history 4

Tissue hypereosinophilia can be defined by one of the following criteria:

Eosinophilic Esophagitis (EoE) is defined as ≥15 eosinophils/high power field (or ≥15 eosinophils/0.3 mm² or >60 eosinophils/mm²) in esophageal biopsies 1
EoE is characterized by symptoms of dysphagia and/or food impaction in adults, and feeding problems, abdominal pain, and/or vomiting in children 1

Helminth infections are the most common identifiable cause of eosinophilia in returning travelers, with diagnosis rates ranging from 19-80% 1, 5
Medications are among the most common non-infectious causes in countries with low rates of endemic helminth infections 5
- Common culprits include NSAIDs, beta-lactam antibiotics, and nitrofurantoin 1, 5
Allergic disorders, including asthma, eczema, and hay fever 1
Myeloproliferative disorders should be considered with eosinophil counts >20,000 cells/μL 2
Other causes include vasculitis, immunodeficiency diseases, and malignancies 2, 6

Even mild persistent eosinophilia can cause end-organ damage if left untreated 1, 5
Eosinophils may directly damage organs, particularly the heart, lungs, and skin, through the release of granule products, lipid mediators, and cytokines 2
Patients with eosinophilia ≥1.5 × 10^9/L for more than 3 months without evidence of end-organ damage should be referred to a hematologist after exclusion of infectious causes 1, 4

Strongyloides stercoralis infection can persist lifelong and later present as hyperinfection syndrome with high mortality in immunocompromised patients 1, 5
Testing for eosinophilia alone is not an adequate screening strategy for helminth infection, as many people with helminth infections do not have eosinophilia 5
In cases of evolving life-threatening end-organ damage, diagnosis and treatment should be expedited 4