What is the significance of an elevated eosinophil count?

Significance of Raised Eosinophil Count

An elevated eosinophil count (>500 cells/μL) most commonly indicates allergic disease or parasitic infection, but the degree of elevation and clinical context determine urgency—mild elevations (<1500 cells/μL) typically reflect allergic conditions, while severe elevations (>5000 cells/μL) warrant immediate investigation for myeloproliferative disorders and potential organ damage. 1

Clinical Interpretation Based on Severity

Mild Eosinophilia (500-1500 cells/μL)

• Allergic disorders are the predominant cause, including asthma (present in 66-100% of patients), allergic rhinitis, and atopic dermatitis 2, 1
• Chronic cough alone can cause eosinophilia in up to 40% of cases 2
• This level rarely indicates serious pathology and typically responds to corticosteroid therapy 2

Moderate Eosinophilia (1500-5000 cells/μL)

• Defined as hypereosinophilia when present on two consecutive tests at least 4 weeks apart 1
• Requires systematic evaluation beyond simple allergic causes 3
• Eosinophilic esophagitis shows this pattern, with 20-100% of children and 10-50% of adults demonstrating elevated counts (typically 2-fold elevation) 2
• Parasitic infections remain a key consideration, particularly with relevant travel history 4

Severe Eosinophilia (>5000 cells/μL)

• Values >20,000 cells/μL are highly suggestive of myeloproliferative disorders and require urgent hematology evaluation 1
• Hypereosinophilic syndrome (HES) must be excluded, as eosinophils can directly damage organs through release of granule products 1, 5
• Cardiac, pulmonary, and cutaneous involvement are the most concerning complications 1

Prognostic and Therapeutic Significance

Predicting Asthma Exacerbations

• Raised sputum eosinophil counts predict asthma exacerbations with 90% sensitivity when corticosteroids are withdrawn 2
• Management strategies targeting eosinophil normalization reduce severe asthma exacerbations by up to 60%, with particular benefit in patients on long-acting beta-agonists or those with severe asthma 2
• A sputum eosinophil count <3% has 100% negative predictive value for significant bronchodilator response 2

Response to Corticosteroid Therapy

• Eosinophil counts decrease 2- to 7-fold with corticosteroid treatment, with effects potentially visible as early as 6 hours 2
• The dose-response curve plateaus at low doses (200 mg/day beclomethasone equivalent), with little additional benefit at higher doses 2
• In eosinophilic esophagitis, 88% of patients show decreased blood eosinophil counts following fluticasone treatment 2

Critical Factors Affecting Interpretation

Temporal and Seasonal Variability

• Eosinophil counts show substantial seasonal variation, with approximately 20% difference between July and January 6
• Counts between 150-299 cells/μL are least stable, with only 44% of patients remaining in the same classification over time 6
• A single measurement may be insufficient for diagnosis or management decisions 6

Confounding Factors

• Corticosteroid therapy significantly reduces eosinophil counts, making recent steroid use a critical historical detail 7
• Current smoking decreases eosinophil levels by 23% 6
• Nasal polyps increase eosinophil levels by 38% 6
• Time from blood collection to analysis affects accuracy, with longer delays reducing counts and increasing variability 6

Organ Damage Assessment

When to Investigate End-Organ Involvement

• Any patient with persistent hypereosinophilia (≥1500 cells/μL) requires evaluation for organ damage 1
• Cardiac involvement is the most life-threatening complication and should be assessed with echocardiography and cardiac biomarkers 1
• Pulmonary function testing and chest imaging are indicated for respiratory symptoms 1
• Skin examination for urticaria, angioedema, or other manifestations is essential 1

Diagnostic Approach Algorithm

Step 1: Confirm True Eosinophilia

• Calculate absolute eosinophil count (percentage × total WBC) to confirm >500 cells/μL 3
• Repeat measurement to exclude laboratory error or transient elevation 1

Step 2: Assess Severity and Timing

• Document onset and any coincident events (travel, new medications, seasonal patterns) 3
• If ≥1500 cells/μL, obtain second measurement 4 weeks later to confirm hypereosinophilia 1

Step 3: Evaluate for Common Causes

• Screen for allergic disorders (asthma, rhinitis, atopic dermatitis) as the most frequent etiology 2, 1
• Obtain travel history and stool examination for parasites 4
• Review all medications for drug hypersensitivity reactions 3

Step 4: Investigate for Serious Pathology if Indicated

• If >5000 cells/μL or unexplained hypereosinophilia, obtain peripheral blood smear, serum tryptase, vitamin B12, and consider bone marrow evaluation for myeloproliferative disorders 4, 5
• Assess for vasculitis and immunodeficiency if other causes excluded 1

Common Pitfalls

• Do not assume allergy alone explains hypereosinophilia (≥1500 cells/μL)—this level always requires further investigation 3
• Peripheral blood eosinophil counts may not correlate with tissue eosinophilia, particularly in eosinophilic esophagitis where tissue biopsy remains the gold standard 2
• Symptoms and eosinophilic inflammation can be dissociated in some asthma phenotypes, with significant discordance seen in secondary care patients 2
• Isolated low or normal eosinophil counts in patients on corticosteroids do not exclude eosinophilic disease—obtain counts before treatment when possible 7