Defining small airways disease- does it matter? Comparison of prevalence and characteristics in the general population based on different definitions

<bold>Rationale:</bold> Small airways disease (SAD) is regarded as a precursor of obstructive airway diseases. The most common used spirometric parameter is the forced expiratory flow between 25% and 75% of vital capacity (FEF25-75). Aim of this study was to compare two definitions in terms of demographic and clinical characteristics in a large general population sample. <bold>Methods:</bold> This study is based on data from 6,906 adults (≥18 years) from the Austrian LEAD study. All individuals had a normal ratio of the forced expiratory volume in the first one second to forced vital capacity equal or above the lower limit of normal (FEV1/FVC≥LLN). Comparison was done between FEF25-75<LLN and FEF25-75/FVC<LLN. <bold>Results:</bold> Prevalence of FEF25-75/FVC defined SAD was higher than FEF25-75 defined SAD (6.7% vs. 1.9%). Subjects with FEF25-75/FVC defined SAD were older (63.8 vs. 45.6 years) and showed variations in body composition parameters including higher mean BMI (26.5 vs. 25.9) and waist circumference (99.5 vs. 95.9cm), lower fat mass index (−0.2 vs. 0.2), and less visceral adipose tissue (−0.2 vs. 0.2). Moreover, they had almost half often a self-reported doctor diagnosis of asthma (12.3% vs. 22.4%) and were less likely to have a family history of asthma (9.8% vs 20.0%). No significant differences were found in the prevalence of current or former smokers (63.0% vs. 57.1%), chronic cough (8.9% vs. 10.6%), and respiratory symptoms (37.8% vs. 44.8%). <bold>Conclusion:</bold> Our findings highlight the importance of how to define SAD. Definition based variations in prevalence, age and body composition will explain variation in presence of cardiometabolic diseases and SAD.