Workup for Eosinophilia
Begin by calculating the absolute eosinophil count (AEC) and stratifying risk: eosinophilia is defined as AEC >0.5 × 10⁹/L, while hypereosinophilia (≥1.5 × 10⁹/L) requires more aggressive investigation and warrants hematology referral if persistent beyond 3 months after excluding infectious causes. 1
Immediate Assessment for End-Organ Damage
Any patient with eosinophilia presenting with cardiac, pulmonary, or neurological symptoms requires urgent medical assessment and consideration of emergency treatment, as persistent eosinophilia can cause significant end-organ damage regardless of the underlying cause 1. This is a critical pitfall to avoid—don't delay workup for symptomatic patients.
Stratified Diagnostic Approach
For Mild Eosinophilia (0.5-1.5 × 10⁹/L)
Focus your history on:
- Medications: The most common non-infectious cause in developed countries alongside allergies 1. Review all current and recently started medications
- Atopic conditions: Asthma, eczema, hay fever 1
- Travel history: Essential for determining helminth exposure risk
- Timing: When did eosinophilia begin and what coincident events occurred 2
For Hypereosinophilia (≥1.5 × 10⁹/L)
Hypereosinophilia is rarely explained by allergy alone and always requires further workup 2. This level demands:
First-line investigations for returning travelers/migrants (per 2025 UK guidelines):
- Helminth serology as the primary test 1
- Filarial investigations only if history of West Africa travel/residence (day/night blood films no longer recommended first-line) 1
- Stool microscopy for parasites
- HIV testing
- Strongyloides serology (critical given hyperinfection risk in immunocompromised patients) 1
Additional essential workup:
- Complete blood count with differential
- Comprehensive metabolic panel
- Vitamin B12 level (elevated in myeloproliferative disorders)
- Tryptase level
- IgE level
- Chest imaging if respiratory symptoms
- Abdominal imaging if gastrointestinal symptoms
For Persistent Hypereosinophilia (≥1.5 × 10⁹/L for >3 months)
Refer to hematology once infectious causes excluded or treated 1. Hematology workup should include:
- Bone marrow examination with morphology assessment 3
- Molecular testing for tyrosine kinase fusions, particularly:
- FIP1L1::PDGFRA (most common, responds dramatically to imatinib) 4
- PDGFRB rearrangements
- FGFR1 rearrangements
- JAK2 mutations
- Flow cytometry to assess for aberrant T-cell populations
- T-cell receptor gene rearrangement studies
Common Etiologies by Context
In travelers/migrants: Helminths cause 19-80% of diagnosed cases 1. Strongyloides and Schistosoma haematobium deserve special attention given their long-term complications.
In non-travelers in developed countries: Allergy/atopy and drugs are most common 1. However, don't anchor on these diagnoses in hypereosinophilia.
Severe eosinophilia (>5 × 10⁹/L): Think myeloproliferative disorders, eosinophilic granulomatosis with polyangiitis, or tissue-migrating parasites 5.
Critical Pitfalls to Avoid
- Don't assume allergy explains hypereosinophilia—this requires further investigation 2
- Don't miss drug-induced eosinophilia: Sulbactam/cefoperazone, meropenem, tazobactam/piperacillin, allopurinol, lamotrigine, and vancomycin are high-risk agents 6, 7
- Don't forget that many helminth infections don't cause eosinophilia—testing eosinophils alone is inadequate screening 1
- Don't delay hematology referral for persistent hypereosinophilia ≥1.5 × 10⁹/L beyond 3 months 1
- Screen for end-organ damage early, particularly cardiac involvement which can be irreversible 1
Treatment Considerations
For asymptomatic eosinophilia in travelers/migrants aged >24 months with negative initial workup, empirical treatment with albendazole 400 mg single dose plus ivermectin 200 μg/kg single dose is now recommended per 2025 UK guidelines 1.
For clonal eosinophilia with PDGFRA rearrangement, imatinib produces rapid normalization (often within 2 weeks) and is the treatment of choice 4.