Introduction: Atopic dermatitis (AD) is a common chronic skin disease with an inflammatory pathophysiology that includes the activation of the innate and adaptive immune systems. We aimed to investigate the neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), eosinophil-to-lymphocyte ratio (ELR), and eosinophil-to-neutrophil ratio (ENR) in AD patients, according to age and disease severity. Methods: This is a retrospective, population-based cohort study conducted between the years 2005 and 2020, comparing hematological markers of AD patients and sex-age-ethnicity-matched controls. AD patients were further divided by age and disease severity (mild, moderate-to-severe AD). We created a decision tree to predict moderate-severe AD. Results: A total of 13,928 patients with AD were included in this study: 6,828 adults and 7,100 children, with 13,548 controls. NLR, PLR, and ELR were lower in children compared to adults (p values <0.001). NLR, PLR, ELR, and ENR were increased in moderate-severe AD patients compared to mild AD patients (p values <0.001). PLR, ELR, and ENR were increased in AD patients versus controls (p values <0.001), with an additional increase in the NLR of moderate-to-severe AD patients. Patients with an ELR <0.21, a PLR >161, and ENR ≤0.016 should be considered high risk for developing severe AD, as well as patients with an ELR >0.21 and age at diagnosis <30 or age >30 years and mean platelet volume ≤9. Conclusion: Hematological ratios were significantly higher in moderate-to-severe AD patients, compared to mild AD patients. Hematological markers were lower in children with AD compared to adults, except for ENR, likely reflecting age-related changes in blood count parameters. These markers can assist in the management and follow-up of AD patients.

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