Besides type 2 inflammation in asthma, what other conditions can cause elevated serum eosinophil counts?

Causes of Elevated Serum Eosinophils Beyond Type 2 Asthma

Elevated serum eosinophils can result from numerous conditions including parasitic infections, drug hypersensitivity reactions, eosinophilic gastrointestinal disorders (particularly eosinophilic esophagitis), vasculitides (especially eosinophilic granulomatosis with polyangiitis/EGPA), hypereosinophilic syndromes (both lymphocytic and myeloproliferative variants), allergic bronchopulmonary aspergillosis, and less commonly HIV infection, IgG4-related disease, and certain malignancies.

Primary Eosinophilic Disorders

Hematological Conditions

• Hypereosinophilic syndromes represent a critical differential, divided into:
  • Lymphocytic variants
  • Myeloproliferative variants (characterized by FIP1L1 fusion genes) 1
• These conditions require specific workup including genetic testing and bone marrow evaluation when eosinophil counts are persistently elevated without clear secondary cause

Vasculitis

• Eosinophilic granulomatosis with polyangiitis (EGPA) is the most important vasculitic cause 1
  • Presents with asthma, eosinophilia, rhinosinusitis, and lung infiltrates
  • 30-40% are ANCA-positive (mostly MPO-ANCA)
  • ANCA-positive patients show more vasculitis features: glomerulonephritis, neuropathy, purpura
  • ANCA-negative patients have more cardiomyopathy and lung involvement
  • Critical pitfall: ANCA are usually NOT present in primary eosinophilic disorders, so ANCA testing helps distinguish EGPA from other causes 1

Gastrointestinal Causes

Eosinophilic Esophagitis (EoE)

• Defined by ≥15 eosinophils per 0.3 mm² on esophageal biopsy with compatible symptoms 2
• Strongly associated with atopic conditions and frequently coexists with asthma 3
• Prevalence has increased dramatically to 63/100,000 since 2017 2
• Shows seasonal variation linked to pollen exposure 2
• Important: Peripheral eosinophilia may be present but diagnosis requires tissue confirmation

Infectious Causes

Parasitic Infections

• Remain a major cause globally of eosinophilia 1
• Must be excluded in appropriate epidemiological contexts before attributing eosinophilia to other causes

Allergic and Hypersensitivity Disorders

Allergic Bronchopulmonary Aspergillosis (ABPA)

• Hypersensitivity reaction to Aspergillus species 1, 4
• Presents with asthma-like symptoms, elevated IgE, and eosinophilia
• Requires specific diagnostic criteria including imaging and serological testing

Drug Hypersensitivity

• Drug reaction with eosinophilia and systemic symptoms (DRESS) 4
• Eosinophilic pneumonia can be drug-induced 5
• Temporal relationship to medication exposure is key diagnostic feature

Pulmonary Eosinophilic Disorders

Eosinophilic Pneumonia

• Idiopathic chronic eosinophilic pneumonia is associated with asthma in most cases 5
• Distinguished from asthma by radiographic infiltrates and higher tissue eosinophilia
• Responds dramatically to systemic corticosteroids

COPD with Eosinophilia

• 10-40% of COPD patients demonstrate eosinophilic inflammation 6
• Sputum eosinophilia predicts corticosteroid responsiveness in both asthma and COPD 6
• Viral infections and inflammatory responses drive eosinophil elevation in eosinophilic COPD 7

Other Conditions

Less Common Causes

• HIV infection 1
• IgG4-related disease 1
• Hypersensitivity pneumonitis 4
• Occupational exposures 4
• Neoplastic disorders 4

Diagnostic Approach

Key Distinguishing Features

When evaluating unexplained eosinophilia:

1. Obtain ANCA testing - positivity (especially MPO-ANCA) suggests EGPA over primary eosinophilic disorders 1

2. Assess for systemic involvement:

   • Cardiac symptoms → consider EGPA (especially if ANCA-negative)
   • Renal involvement, neuropathy, purpura → consider EGPA (especially if ANCA-positive)
   • Dysphagia/food impaction → obtain esophageal biopsies for EoE 2

3. Evaluate medication history for drug-induced eosinophilia 4

4. Consider parasitic infection based on travel/exposure history 1

5. Look for FIP1L1 fusion genes if myeloproliferative hypereosinophilic syndrome suspected 1

Critical Caveat

Corticosteroid exposure significantly affects blood eosinophil counts 8, so interpretation must account for recent or ongoing steroid therapy. Longitudinal measurements provide more reliable assessment than single values.