Combining small airway parameters with conventional parameters obtained during spirometry to diagnose airflow obstruction: A cross-sectional study.

BACKGROUND AND OBJECTIVE: The use of small airway parameters generated by spirometry, namely forced expiratory flow between 25% and 75% of forced vital capacity (FVC) (FEF25%-75% ) and forced expiratory flow at 50% and 75% of FVC (FEF50% and FEF75% , respectively), is widely discussed. We evaluated the importance of these spirometric parameters in a large Chinese population.

METHODS: We conducted a cross-sectional observational study in which spirometry and bronchodilator responsiveness (BDR) data were collected in a healthcare centre from May 2021 to August 2022 and in a tertiary hospital from January 2017 to March 2022. Discordance was assessed between the classification of test results by the large airway parameters of forced expiratory volume in 1 second (FEV1 ) and FEV1 /FVC ratio and by the small airway parameters of FEF25%-75% , FEF75% and FEF50% . The predictive power of Z-scores of spirometric parameters for airflow limitation and BDR was assessed using receiver operating characteristic curves.

RESULTS: Our study included 26,658 people. Among people with a normal FVC (n = 14,688), 3.7%, 4.5% and 3.6% of cases exhibited normal FEV1 /FVC ratio but impaired FEF25%-75% , FEF75% and FEF50% , respectively, while 6.8%-7.0% of people exhibited normal FEV1 but impaired FEF25%-75% , FEF75% and FEF50% . Using the Z-scores of combining both large and small airway parameters in spirometry showed the best area under the curve for predicting airflow limitation (0.90; 95% CI 0.87-0.94) and predicting BDR (0.72; 95% CI 0.71-0.73).

CONCLUSION: It is important to consider both large and small airway parameters in spirometry to avoid missing a diagnosis of airflow obstruction.