Association between the imbalance in peripheral lymphocyte subsets and pneumonia-associated acute respiratory distress syndrome in adults: a cross-sectional study

Background Acute respiratory distress syndrome (ARDS) is a severe complication of pneumonia that significantly increases mortality. Its pathogenesis is associated with dysregulated host immune responses, which has been a topic of global research in lung injury. Methods In this cross-sectional study, we enrolled adult patients with pneumonia from the Department of Infectious Diseases between May 2021 and June 2025. Participants were categorized into ARDS and non-ARDS groups based on the presence of ARDS at admission. We employed multivariable logistic regression and restricted cubic spline models to evaluate the association between lymphocyte subset levels and ARDS, adjusting for key clinical confounders. Results Of the 227 eligible patients, 127 were diagnosed with ARDS. Patients with ARDS exhibited lower CD3 + CD4 + T-cell counts compared to non-ARDS controls. Furthermore, CD3 + CD4 + T-cell counts decreased progressively with increasing ARDS severity ( P for trend <0.001). After comprehensive adjustment, a twofold higher CD3 + CD4 + T-cell count was significantly associated with 33% lower odds of presenting with ARDS (adjusted OR 0.67, 95% CI 0.48–0.95), with evidence of a dose–response relationship. A nonlinear threshold effect was observed, with a significant inverse association noted above 227 cells/μL. This relationship remained consistent across all prespecified subgroups, with no evidence of effect modification. Conclusion This study indicates an independent inverse association between CD3 + CD4 + T-cell counts and the presence of pneumonia-related ARDS. However, the cross-sectional design precludes any conclusions about its clinical applicability. Further research in prospective cohorts is warranted.