Background A substantial proportion of individuals with airflow limitation are never-smokers. However, whether never-smokers with airflow limitation have undiagnosed asthma is unknown. We hypothesised that the majority of never-smokers with respiratory symptoms and airflow limitation but without known asthma have undiagnosed asthma by comparing characteristics and prognosis in never-smokers with airflow limitation and asthma (NS+AFL+A) with never-smokers with airflow limitation but without asthma (NS+AFL−A).

Methods Among 94 079 participants aged 20–100 years from the general population, 39 102 (42%) were never-smokers. In this group, 13 719 (35%) reported to have respiratory symptoms of whom 1610 (12%) had airflow limitation. We investigated characteristics and risk of complications (asthma or COPD exacerbations, pneumonias and all-cause mortality) and comorbidities (lung cancer, ischaemic heart disease, myocardial infarction, deep venous thrombosis and PE) during 4.5 years median follow-up.

Results NS+AFL−A compared with NS+AFL+A reported less allergy and respiratory symptoms, and had higher FEV₁ and lower levels of eosinophils and IgE in peripheral blood. NS+AFL+A had increased risk of asthma and COPD exacerbations, but not of pneumonias; adjusted HRs in NS+AFL+A compared with NS+AFL−A were 16 (95% CI 3.7 to 73) for asthma exacerbations and 15 (2.8 to 80) for COPD exacerbations. Still, NS+AFL−A had increased risk of COPD exacerbations and pneumonias, but not of asthma exacerbations; adjusted HRs in NS+AFL−A compared with never-smokers without airflow limitation or asthma (NS−AFL−A) were 7.7 (2.8 to 21) for COPD exacerbations and 1.7 (1.3 to 2.3) for pneumonias. Risk of comorbidities or all-cause mortality was not increased in NS+AFL−A or NS+AFL+A compared with NS−AFL−A.

Conclusions Majority of NS+AFL−A do not seem to have undiagnosed asthma and may instead have airflow limitation caused by other risk factors.