Association between NPAR and asthma–COPD overlap: a nonlinear analysis of NHANES data

Abstract

Background
Asthma-COPD Overlap (ACO) is a chronic respiratory disorder characterized by features of both asthma and chronic obstructive pulmonary disease (COPD). The neutrophil-to-albumin ratio (NPAR) serves as an indicator of systemic inflammation and nutritional status; however, its relationship with ACO has not yet been clearly established.


Methods
Data were obtained from the 2015–2018 national health and nutrition examination survey and restricted to adults aged 45 years and older. Multivariable logistic regression models were employed to assess the association between NPAR and ACO, adjusting for potential confounders. To evaluate non-linearity, smooth curve fitting and a two-piecewise linear regression model were applied to identify potential threshold effects. Subgroup analyses were further conducted to investigate population heterogeneity.


Results

A total of 2,611 participants were included, with a median age of 62 years and an ACO prevalence of 2.45%. In the fully adjusted model (Model III), each 100-unit increase in NPAR was associated with higher odds of ACO (OR = 1.017, 95% CI: 1.001–1.033,
p
 = 0.040). A non-linear relationship was observed, with an inflection point at NPAR = 13.23. Subgroup analyses revealed heterogeneity by age, education level, and drinking status. Specifically, for age groups 45–59 years and 60–79 years, the odds ratios (ORs) were 1.023 (95% CI: 1.002–1.045,
p
 = 0.035) and 1.014 (95% CI: 1.000-1.029,
p
 = 0.049), respectively. For drinking status, participants consuming 1–5 drinks per month and those who never drank had ORs of 0.932 (95% CI: 0.875–0.993,
p
 = 0.029) and 1.027 (95% CI: 1.002–1.052,
p
 = 0.033), respectively. Regarding education level, participants with less than a 9th-grade education and those with some college or an AA degree had ORs of 0.932 (95% CI: 0.875–0.993,
p
 = 0.029) and 1.039 (95% CI: 1.018–1.060,
p
 < 0.001), respectively.



Conclusion
A higher NPAR index was independently associated with a lower risk of ACO, highlighting its potential utility as a biomarker for risk assessment in clinical practice. By reflecting systemic inflammation, immune function, and nutritional status, the NPAR index provides a novel approach for the early identification of individuals at high risk of ACO.