Comparison of the lower limit of normal to the fixed ratio method for the diagnosis of airflow obstruction at high altitudes: a large cross-sectional survey of subjects living between 3000-4700 m above sea level

Background: There is no general agreement on the preferential use of a fixed ratio (FR) of forced expiratory volume in 1 s (FEV 1 )/forced vital capacity (FVC) < 0.7 vs. the lower limit of normal (LLN) of FEV 1 /FVC to define airflow obstruction. Determining the impact of these different cut-off levels in people living at high altitudes has not been studied. We assessed the prevalence of airflow obstruction and its clinical characteristics in residents living at high altitude using a fixed ratio and the LLN of FEV 1 /FVC according to Global Lung Initiative 2012 (GLI) reference values. Methods: Using a multistage stratified sampling method, 3702 participants (aged ≥ 15 years) living at an altitude of 3000-4700 m in Tibet were included. Results: 11.4% and 7.7% of participants had airflow obstruction according to GLI-LLN and a fixed FEV 1 /FVC cut-off value, respectively. The participants in the FR-/LLN+ group were younger, predominantly female, more frequently exposed to household air pollution, and had a higher proportion of chronic obstructive pulmonary disease assessment test scores ≥ 10 than those in the FR-/LLN- group. They also had a significantly lower FEV 1 and a higher frequency of small airway dysfunction. Compared with the participants of the FR+/LLN+ group, those in the FR-/LLN+ group showed no significant difference in the risk factors for airflow obstruction and respiratory symptoms, but had a lower prevalence of small airway dysfunction. Conclusions: Defining airflow obstruction according to LLN, instead of using an FR, identified younger individuals with more frequent clinical symptoms of airflow obstruction and small airway dysfunction.