Cutoff for Eosinophilia
Eosinophilia is defined as an absolute eosinophil count (AEC) >500 cells/μL in peripheral blood, while hypereosinophilia is defined as an AEC ≥1500 cells/μL. 1, 2
Standard Definitions for Peripheral Blood
- Mild eosinophilia: AEC 500-1500 cells/μL 3
- Hypereosinophilia: AEC ≥1500 cells/μL 4, 5, 6
- Normal range: 50-500 cells/μL 7
The distinction between eosinophilia and hypereosinophilia is clinically meaningful because it guides the diagnostic approach. Mild eosinophilia (500-1500 cells/μL) is commonly explained by allergic conditions, parasitic infections, or medication reactions. 1 However, hypereosinophilia (≥1500 cells/μL) is rarely explained by allergy alone and should always prompt a comprehensive workup for primary hematologic disorders, end-organ damage, and other serious underlying conditions. 1
Tissue-Specific Cutoffs
Esophageal Tissue (Eosinophilic Esophagitis)
For eosinophilic esophagitis diagnosis, the cutoff is ≥15 eosinophils per 0.3 mm² (equivalent to one high-power field) in esophageal biopsy specimens. 8, 9, 7
- This represents the peak count (highest density area), not an average across all fields 8, 7
- The 0.3 mm² standardization addresses historical variability in microscope field sizes 7
- At least one high-power field must contain ≥15 eosinophils, even if the mean across all fields is lower due to patchy inflammation 8
- Histological remission after treatment is defined as <15 eosinophils per 0.3 mm² 9
- Deep remission is defined as <5 eosinophils per 0.3 mm² 9
The 2024 British Society of Gastroenterology guidelines emphasize that this 15 eosinophils/0.3 mm² threshold has 100% sensitivity and 96% specificity for distinguishing eosinophilic esophagitis from gastroesophageal reflux disease (which typically shows <5 eosinophils per high-power field). 8
Blood Eosinophilia in ABPA Context
For allergic bronchopulmonary aspergillosis (ABPA) evaluation, the 2024 ISHAM-ABPA Working Group recommends a blood eosinophil cutoff of 500 cells/μL to guide therapy decisions, including initiation of anti-type 2 biological agents or combination therapy. 8
Important Clinical Caveats
Peripheral blood eosinophilia does not always correlate with tissue eosinophilia. Only 10-50% of adults and 20-100% of children with eosinophilic esophagitis have elevated peripheral eosinophil counts, and these are typically only modest 2-fold elevations. 9, 7
Hypereosinophilic syndrome requires both sustained eosinophilia (AEC ≥1500 cells/μL for at least 6 months) AND documented target organ damage (heart, lung, skin, or nerve tissue). 4 This is a diagnosis of exclusion after ruling out secondary and clonal causes.
Even mild persistent eosinophilia can cause end-organ damage if left untreated, and some helminth infections can persist lifelong with potential for hyperinfection syndrome in immunocompromised patients. 3
Testing for eosinophilia alone is not adequate for screening helminth infections, as many infected individuals do not have eosinophilia. 3