AbstractRationaleChurg–Strauss syndrome (CSS) and hypereosinophilic syndrome (HES) overlap considerably in clinical presentation. A reliable means of distinguishing between these groups of patients is needed, especially in the setting of glucocorticoid therapy.MethodsA retrospective chart review of 276 adult subjects referred for evaluation of eosinophilia > 1500/μl was performed, and subjects with a documented secondary cause of eosinophilia or a PDGFR ‐positive myeloproliferative neoplasm were excluded. The remaining subjects were assessed for the presence of American College of Rheumatology (ACR) criteria. Laboratory and clinical parameters were compared between subjects with biopsy‐proven vasculitis (CSS; n = 8), ≥4 ACR criteria (probable CSS; n = 21), HES with asthma and/or sinusitis without other CSS‐defining criteria (HESwAS; n = 20), HES without asthma or sinusitis (HES; n = 18), and normal controls (n = 8). Serum biomarkers reported to be associated with CSS were measured using standard techniques.ResultsThere were no differences between the subjects with definite or probable CSS or HES with respect to age, gender, or maintenance steroid dose. Serum CCL17, IL‐8, and eotaxin levels were significantly increased in eosinophilic subjects as compared to normal controls, but were similar between the eosinophilic groups. Serum CCL17 correlated with eosinophil count (P < 0.0001, r = 0.73), but not with prednisone dose.ConclusionsIn patients with a history of asthma and sinusitis, distinguishing between ANCA‐negative CSS and PDGFR ‐ne‐gative HES is difficult because of significant overlap in clinical presentation and biomarker profiles.