Sex-specific associations of serum retinol-binding protein 4 with type 2 inflammatory biomarkers and pulmonary function in adults with asthma

Background Retinol-binding protein 4 (RBP4) has been linked to immune regulation, but whether its associations with type 2 inflammatory biomarkers (eosinophils and immunoglobulin E (IgE)) and pulmonary function differ by sex in asthma remains unknown. Objectives To evaluate the sex-specific associations of serum RBP4 with type 2 inflammatory biomarkers and spirometric indices in adults with asthma. Methods Adults with physician-diagnosed asthma attending a respiratory clinic between January and June 2026 were consecutively enrolled in this cross-sectional study. Associations of serum RBP4 with inflammatory biomarkers and spirometric indices were evaluated using Spearman’s and Pearson’s correlation analyses, as appropriate. Results A total of 88 adults with asthma (35 males and 53 females) were included. Females had a higher body mass index and FEV₁/FVC ratio than males (both P < 0.01), whereas other baseline characteristics were comparable. Serum RBP4 was positively correlated with total IgE in both males (ρ = 0.66, P < 0.01) and females (ρ = 0.53, P < 0.01), but not with peripheral blood eosinophil count in either sex. Serum RBP4 was not associated with FEV₁/FVC or PEF in males or females. A modest inverse correlation between serum RBP4 and FEF₂₅–₇₅ was observed only in males ( r = − 0.35, P = 0.03), with no corresponding association in females. Conclusion Serum RBP4 was positively associated with total IgE, but not peripheral blood eosinophils or conventional spirometric indices, although a modest male-specific inverse association with FEF₂₅–₇₅ warrants confirmation in larger prospective studies.