Normal Eosinophil Counts in Adults
The upper limit of normal for blood eosinophil count in healthy adults is 300-400 cells/μL (or 0.3-0.4 × 10⁹/L), varying slightly by age. 1
Reference Ranges by Population
General Adult Population
- The 95th percentile (upper limit of normal) ranges from 400-500 cells/μL in unselected adults 1
- In adults without asthma, COPD, or clinical allergy, the upper limit is 300-400 cells/μL 1
- Community-based studies of normal subjects show the upper limit for sputum eosinophil differential count is 1.9% 2
Clinical Definition Thresholds
- Eosinophilia is defined as absolute eosinophil count >500 cells/μL 3, 4
- Mild eosinophilia is classified as 0.5-1.5 × 10⁹/L (500-1,500 cells/μL) 5, 6
- Hypereosinophilia is defined as ≥1.5 × 10⁹/L (≥1,500 cells/μL) 5, 4
Factors That Influence Normal Ranges
Age-Related Variation
- The upper limit of normal BEC varies by age, with sputum neutrophil counts increasing particularly over age 50 2
- Age correction factors have been suggested for both normal subjects and asthma patients 2
- Reference values should account for age-specific variations 1
Conditions That Elevate Baseline Counts
- Asthma significantly increases eosinophil levels in multivariable analysis 1
- Clinical allergy (atopy) elevates baseline counts 1
- Obesity raises eosinophil levels in those without respiratory disease 1
- Diabetes is associated with higher baseline counts 1
- Nasal polyps increase eosinophil levels by 38% 7
Factors That Lower Counts
- Current smoking decreases eosinophil levels by 23% 7
Important Clinical Considerations
Variability and Stability
- Eosinophil counts show substantial seasonal variation, with approximately 20% difference between July and January 7
- Counts between 150-299 cells/μL are least stable, with only 44% of patients remaining in the same classification over time 7
- A single measurement may not be sufficient for clinical decision-making 7
Technical Factors
- Time from blood collection to analysis affects results, with longer delays reducing measured counts 7
- Laboratory-to-laboratory differences exist, though correlation is generally high (R=0.89) 7
- Variability increases with higher absolute eosinophil counts 7
Distribution Characteristics
- In normal subjects, differential counts of each cell type follow an approximately normal distribution 2
- In disease states like asthma, eosinophil counts tend to be right-skewed, requiring log-transformation for statistical analysis 2
Common Pitfalls to Avoid
- Do not assume a single borderline elevated count represents true eosinophilia without repeat testing, given the substantial temporal variability 7
- Do not interpret eosinophil counts without considering smoking status, atopic conditions, obesity, and seasonal timing 7, 1
- Do not rely solely on peripheral blood counts to assess tissue eosinophilia in conditions like eosinophilic esophagitis, where tissue biopsy remains the gold standard 6, 8
- Do not use eosinophilia alone as adequate screening for helminth infection, as many infected patients have normal counts 5, 8