Eosinophilia Definition
Eosinophilia is defined as an absolute eosinophil count (AEC) greater than 0.5 × 10⁹/L (500 cells/μL). 1
Standard Cutoff Values
The most recent UK guidelines (2025) for investigation and management of eosinophilia establish >0.5 × 10⁹/L (>500 cells/μL) as the diagnostic threshold 1. This cutoff is consistently supported across multiple research studies 2, 3, making it the universally accepted standard in clinical practice.
Clinical Severity Stratification
Beyond the basic definition, eosinophilia severity is categorized as follows:
- Mild eosinophilia: 500-1,499 cells/μL
- Moderate eosinophilia (hypereosinophilia): 1,500-4,999 cells/μL
- Severe eosinophilia: ≥5,000 cells/μL 4, 5
Hypereosinophilia specifically refers to AEC ≥1.5 × 10⁹/L (≥1,500 cells/μL) 2, 5, which is an important distinction because this level rarely results from allergy alone and mandates more aggressive workup for serious underlying conditions.
Critical Clinical Thresholds
For patients with AEC ≥1.5 × 10⁹/L persisting for more than 3 months without evidence of end-organ damage, referral to hematology is recommended once infectious causes have been excluded or treated 1. This threshold separates benign reactive eosinophilia from potentially dangerous hypereosinophilic conditions.
Recent evidence (2026) demonstrates that AEC >30,000/μL is rarely due to parasitic infection and should prompt expedited evaluation for non-infectious causes, particularly malignancy and hypereosinophilic syndromes 6. Only 2 patients with parasitic infections in a large cohort exceeded this threshold, both children with visceral larva migrans.
Common Pitfall
A critical caveat: Many helminth infections do not cause eosinophilia, so testing for eosinophilia alone is inadequate as a screening strategy for parasitic infection 1. The absence of eosinophilia does not exclude parasitic disease in at-risk populations.