Background Hypereosinophilia syndrome (HES) is a group of diseases characterized by abnormal increase of eosinophil in the blood and/or tissues, with heterogeneous clinical manifestations, which is easy to be missed or misdiagnosed as other eosinophilia-associated pruritic dermatitis, in which differential diagnosis with atopic dermatitis (AD) is required.

Objective To evaluate the role of altered peripheral blood eosinophil counts in the differentiation of hypereosinophilia syndrome from AD.

Methods Patients with definite diagnosis of HES and AD admitted to the Department of Dermatology of the First Medical Center of Chinese PLA General Hospital from January 2015 to December 2022 were used as the samples for constructing the diagnostic model. The area under the curve (AUC) was calculated and differential diagnostic efficacy was evaluated by the receiver operation characteristic (ROC) method. Patients with definite diagnosis of HES and AD patients with peripheral blood eosinophil counts >0.5 × 10⁹/L admitted from June 2023 to December 2023 were used as the follow-up study subjects to compare the trends of 2-week peripheral blood eosinophil counts after 2-week non-hormonal systemic therapy in both patients.

Results There were 58 cases in the HES group, including 49 males and 9 females, with median age of 55 (IQR: 23-67) years old; and 133 cases in the AD group, including 74 males and 59 females, with age of 55 (IQR: 38-69) years old. Patients in the HES group were older and had more males (P < 0.05). Furthermore, white blood cells counts M(IQR): 10.78 (8.97-13.75) × 10⁹/L vs 6.61(5.44-8.55) × 10⁹/L and peripheral blood eosinophil counts M(IQR): 3.29 (1.77-5.15 ) × 10⁹/L vs 0.6(0.35-1.05) × 10⁹/L in the HES group were significantly higher than those in the AD group (P < 0.05). AUC of EOS to differentially diagnose HES and AD was 0.944 (95% CI: 0.910-0.977), with the optimal threshold was 1.52 × 10⁹/L, sensitivity of 83.2% and specificity of 91.8%. In the follow-up study, the mean value of 10 HES patients EOS was (4.51 ± 1.22) × 10⁹/L before treatment, and after 2 weeks of non-hormonal systemic treatment, the mean value of EOS was (5.56 ± 1.31) × 10⁹/L, without significant decreasing trend. After non-hormonal systemic treatment, the 10 AD patients EOS decreased to normal level (<0.5 × 10⁹/L) on about the 4th-8th day, and the degree of decline was (72.10 ± 17.12)%.

Conclusion HES and AD eosinophilic distribution range is not equal, and the majority of patients with AD receive a nonhormonal systemic treatment in a short period of time, the peripheral blood eosinophilic counts decrease significantly, which can be used as a reference for the preliminary differential diagnosis of HES and AD. The intervention therapy can be initiated rapidly to reduce eosinophil infiltration in HES before irreversible organ damage occurs, thereby reducing the risk of organ damage.