What are the causes and treatments for elevated eosinophil (EOS) counts?

Elevated Eosinophil Count: Causes and Management

Begin by determining the absolute eosinophil count (AEC) and categorizing severity: mild (500-1500 cells/μL), moderate (1500-5000 cells/μL), or severe (>5000 cells/μL), as this stratification directly guides your diagnostic approach and urgency of workup. 1, 2, 3

Immediate Assessment

For any patient with eosinophilia ≥1500 cells/μL persisting more than 3 months, refer urgently to hematology and assess for end-organ damage affecting the heart, lungs, and central nervous system. 1, 2

• Obtain echocardiography or cardiac MRI, pulmonary function tests, chest imaging, and neurological evaluation if hypereosinophilia is present 2
• Values >20,000 cells/μL are highly suggestive of myeloproliferative disorders and require immediate hematology consultation 3

Primary Causes by Frequency

Allergic Disorders (Most Common - ~80% of cases)

Allergic conditions including asthma, allergic rhinitis, atopic dermatitis, and drug reactions account for the vast majority of eosinophilia cases and typically present with mild elevations. 2, 3

• 50-80% of patients with eosinophilic esophagitis have concurrent atopic conditions that may independently contribute to peripheral eosinophilia 2
• Chronic cough alone causes eosinophilia in up to 40% of cases 4
• Review all medications comprehensively, including recent additions or changes, as drug reactions are a leading cause 2
• Immune checkpoint inhibitor therapy can cause eosinophilia as part of immune-related adverse events 2

Parasitic Infections (Second Most Common)

Helminths are the commonest identifiable cause in returning travelers (19-80% of diagnosed cases) and migrants (12-31% prevalence), with Strongyloides stercoralis requiring particular attention due to risk of fatal hyperinfection syndrome in immunocompromised patients. 1, 2

• Obtain stool microscopy for ova and parasites plus serology if gastrointestinal symptoms or travel history is present 2, 5
• For asymptomatic eosinophilia in returning travelers aged >24 months, treat empirically with albendazole 400 mg single dose plus ivermectin 200 μg/kg single dose. 1, 5
• Schistosoma haematobium is associated with squamous cell bladder carcinoma and requires treatment with praziquantel 40 mg/kg in two divided doses for 1 day 1, 5

Essential Diagnostic Testing

Obtain complete blood count with differential, serum chemistries, vitamin B12 levels, serum immunoglobulin levels, inflammatory markers (CRP, ESR), and urinalysis with protein assessment for all patients with confirmed eosinophilia. 2

Travel History Specific Testing

• Filarial investigations are now only recommended for those with history of travel to or residence in West Africa 1
• Serology is the sole first-line test for filarial infections; day/night blood samples are no longer recommended 1

Tissue-Specific Eosinophilic Disorders

Eosinophilic Esophagitis

Peripheral blood eosinophilia occurs in only 10-50% of adults and 20-100% of children with eosinophilic esophagitis, typically with modest 2-fold elevations, making tissue biopsy the gold standard for diagnosis. 2, 4

• Diagnosis requires ≥15 eosinophils per high-power field on esophageal biopsy plus symptoms of esophageal dysfunction 1, 2
• Must exclude GERD through normal pH monitoring or lack of response to high-dose PPI treatment before diagnosing eosinophilic esophagitis 1, 2
• Treat with PPI therapy twice daily for 8-12 weeks, topical steroids, or two-food elimination diet (milk +/- wheat or egg) with dietitian support. 5
• 88% of patients show decreased blood eosinophil counts following fluticasone treatment 4

Myeloid/Lymphoid Neoplasms

For suspected neoplastic eosinophilia, obtain bone marrow biopsy with cytogenetics, FISH, and RT-PCR to evaluate for tyrosine kinase fusion genes (PDGFRA, PDGFRB, FGFR1, JAK2). 2

• Elevated serum tryptase and vitamin B12 levels are commonly observed alongside eosinophilia in these conditions 4
• The eosinophilia is driven by clonal expansion rather than allergic mechanisms 4

Eosinophilic Granulomatosis with Polyangiitis (EGPA)

Consider EGPA in patients presenting with asthma, nasal polyposis, and systemic vasculitis, though only 30-40% are ANCA-positive. 2

Treatment Algorithm by Etiology

For Parasitic Infections

• Albendazole 400 mg single dose for most helminth infections 5
• Ivermectin 200 μg/kg daily for 1-2 days for strongyloidiasis 5
• Praziquantel 40 mg/kg in two divided doses for schistosomiasis 5

For Hypereosinophilic Syndrome

• Corticosteroids are first-line therapy, with eosinophil counts decreasing 2- to 7-fold, with effects potentially visible as early as 6 hours. 4
• The dose-response curve plateaus at low doses 4

For Allergic Disorders

• Corticosteroid therapy typically resolves mild eosinophilia from allergic causes 4
• Management strategies targeting eosinophil normalization reduce severe asthma exacerbations by up to 60% 4

Critical Pitfalls to Avoid

Many people with helminth infection do not have eosinophilia, so testing for eosinophilia alone is not an adequate screening strategy for helminth infection. 1, 5

• Peripheral blood eosinophil counts may not correlate with tissue eosinophilia, particularly in eosinophilic esophagitis where tissue biopsy remains the gold standard 2, 4
• Allergy testing to foods is not recommended for choosing dietary restriction therapy for eosinophilic esophagitis 5
• Hypereosinophilia is rarely, if ever, explained by allergy alone and should always prompt further workup 6
• Symptoms and eosinophilic inflammation can be dissociated in some asthma phenotypes 4

Follow-Up Monitoring

Obtain follow-up eosinophil counts after treatment to assess response, and for persistent eosinophilia despite treatment, refer to hematology for further investigation. 5

• For eosinophilic esophagitis, histological remission is defined as <15 eosinophils per 0.3 mm² in tissue biopsies 5
• Raised sputum eosinophil counts predict asthma exacerbations with 90% sensitivity when corticosteroids are withdrawn 4