Eosinophilia

What is Eosinophilia?

Eosinophilia refers to an eosinophil count > 0.5 ×10⁹/L. It can be reactive to infection, allergy, autoimmune conditions, or reflect a primary haematological disorder.

Common Causes

  • Allergic disorders (e.g. asthma, eczema, hay fever)
  • Parasitic infections (especially helminths)
  • Drug reactions (e.g. antibiotics, NSAIDs)
  • Autoimmune diseases (e.g. eosinophilic granulomatosis with polyangiitis)
  • Haematological malignancies (e.g. Hodgkin lymphoma, chronic eosinophilic leukaemia)
  • Skin diseases (e.g. pemphigoid)

Stepwise Clinical Approach

Step 1: Confirm and Repeat

Confirm persistent eosinophilia over 1-2 repeat samples to exclude transient/reactive causes.

When repeating, also request:

  • CRP and ESR: to screen for inflammation
  • Strongyloides serology (if travel risk): to detect latent parasitic infection
  • Autoimmune screen (e.g. ANCA): if vasculitis suspected
  • Consider CXR (rare): to look for pulmonary infiltrates (e.g. eosinophilic lung disease)
  • Blood film: to check for abnormal eosinophils or blasts

These tests help differentiate between benign, reactive, and malignant causes.

Step 2: Identify Reactive Causes

  • Recent allergic reaction or new medication?
  • Travel history suggesting parasitic exposure?
  • Known history of atopic conditions (e.g. asthma, eczema)?

If a reactive cause is identified, treat accordingly and monitor resolution.

Step 3: Interpretation Based on Results

  • Mild eosinophilia (0.5-1.5 ×10⁹/L): often allergic or mild reactive
  • Moderate eosinophilia (1.5-5.0 ×10⁹/L): consider parasitic infection or autoimmune disease
  • Severe eosinophilia (> 5.0 ×10⁹/L): consider haematological malignancy or hypereosinophilic syndrome

Step 4: Monitor or Refer

  • Mild reactive eosinophilia: monitor every 3-6 months if clinically stable
  • No obvious cause or moderate eosinophilia: consider specialist referral (e.g. infectious diseases, rheumatology)
  • Severe or unexplained eosinophilia: urgent referral to haematology

Step 5: Red Flags Requiring Urgent Action

  • Severe eosinophilia (> 5.0 ×10⁹/L)
  • Signs of organ damage (e.g. cardiac, lung, neurological involvement)
  • Constitutional symptoms (e.g. unexplained weight loss, night fevers)
  • Blood film abnormalities or blasts

Urgent haematology referral is indicated if any red flags are present.

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