Redefining Hematology Referral Thresholds for Eosinophilia

ABSTRACT Objectives Eosinophilia is a common laboratory finding that frequently prompts referral for hematologic evaluation, yet the clinical utility of these consultations remains poorly characterized. This study aims to refine stratification for eosinophilia and characterize the spectrum of associated diagnoses. Methods We conducted a retrospective study of patients referred to hematology at the Oregon Health & Science University between 2015 and 2025 with confirmed eosinophilia based on an absolute eosinophil count (AEC) ≥ 0.5 × 10 9 /L. Results A total of 142 patients were evaluated for eosinophilia during the study period. The etiology of eosinophilia was deemed idiopathic in 44.8% ( n = 64) of patients. Hypereosinophilic syndrome was diagnosed in 20.3% of patients ( n = 29) and analyzed as a clinical syndrome, with most cases classified as HES not otherwise specified. The identifiable secondary causes of eosinophilia included asthma (9.8%), parasitic infection (7.7%), autoimmune disease (4.9%), and drug reactions (4.9%). Eosinophilia‐specific therapies were administered in 22.5% ( n = 32) of patients. Receiver operating characteristic (ROC) analysis demonstrated that peak AEC was a strong predictor of clinically meaningful eosinophilia, defined as a diagnosis of HES or need for eosinophilia‐specific treatment, with an area under the curve of 0.83. An AEC threshold of 1.5 × 10 9 /L yielded a high sensitivity of 97% for clinically meaningful eosinophilia. Conclusions These findings support considering a threshold of 1.5 × 10 9 /L or higher for hematology referral, especially in low‐risk and asymptomatic patients, though external validation is needed.