Evaluation and differential diagnosis of eosinophilia: A tertiary allergy center experience

Eosinophilia is a hematologic finding that may indicate a wide range of underlying conditions, from common allergic diseases to rare and potentially life-threatening disorders such as hypereosinophilic syndrome (HES), eosinophilic granulomatosis with polyangiitis (EGPA), and systemic mastocytosis. A structured diagnostic approach is crucial to identify the underlying etiology and guide appropriate treatment.

This retrospective cross-sectional study evaluated 232 adult patients with peripheral blood absolute eosinophil counts ≥1000 cells/µL, who were referred to a tertiary immunology and allergy clinic between January 2018 and December 2022. Demographic, clinical, and laboratory data were analysed. Diagnoses were grouped according to eosinophil count severity: Mild (1000-1499 cells/µL), moderate (1500-4999 cells/µL), and severe (≥5000 cells/µL). Diagnoses were based on established international guidelines, supported by laboratory, radiological, immunological, and genetic investigations.

The most commonly observed diagnoses included atopic diseases (53.4%), with allergic bronchopulmonary aspergillosis (ABPA, 11.6%), nonsteroidal anti-inflammatory drug-exacerbated respiratory disease (6%), and chronic eosinophilic pneumonia (3%) occurring less frequently. Rare conditions included EGPA (1.7%), HES (1.7%), parasitic infections (1.3%), systemic mastocytosis (0.4%), and Gleich syndrome (0.4%). Innate immune defect was detected in 0.4%. Molecular testing identified myeloid variant HES in four patients (FIP1L1-PDGFRA and PDGFRB mutations). The etiology remained undetermined in 19.8% of the patients. Atopic diseases were the most frequent diagnosis across all eosinophilia severity groups (38.3% vs. 14.2% vs. 0.8% p< 0.001 respectively).

Atopic diseases are the most common cause of eosinophilia in adults, but clinicians must remain vigilant for less common but clinically significant diagnoses such as ABPA, HES, EGPA, and systemic mastocytosis. Managing eosinophilia effectively depends on a multidisciplinary strategy that incorporates algorithm-based decision-making to support accurate diagnosis.