[The cutoff value of small airway dysfunction in children with bronchial asthma]

Objective: To explore the cutoff value for assessing small airway dysfunction in children with asthma. Methods: A total of 364 asthmatic children aged 5 to 14 years, with normal ventilatory function, followed up at the Asthma Clinic of the Children's Hospital of Capital Institute of Pediatrics from January 2017 to January 2018, were selected as the case group. Concurrently, 403 healthy children of the same age range and without any symptoms in the community were chosen as the control group, and pulmonary function tests were conducted. The values of forced expiratory volume in 1 second (FEV₁), forced vital capacity (FVC), forced expiratory flow at 50% of FVC (FEF₅₀), forced expiratory flow at 75% of FVC (FEF₇₅) and maximum mid-expiratory flow (MMEF) were compared between case group and control group. Statistical tests such as t-test, χ² test, or Mann-Whitney U test were used to analyze the differences between the groups. Receiver operating characteristic (ROC) curves were constructed, and the maximum Youden Index was utilized to determine the optimal cutoff values and thresholds for identifying small airway dysfunction in asthmatic children. Results: This study comprised 364 children in the case group (220 boys and 144 girls) and 403 children in the control group (198 boys and 205 girls). The small airway parameters (FEF₅₀%pred, FEF₇₅%pred, MMEF%pred) in the asthmatic group were significantly lower than in the control group (77% (69%, 91%) vs. 95% (83%, 109%), 67% (54%, 82%) vs. 84% (70%, 102%), 76% (66%, 90%) vs. 97% (86%, 113%), Z=12.03, 11.35, 13.66, all P<0.001). The ROC curve area under the curve for FEF₅₀%pred, FEF₇₅%pred, MMEF%pred was 0.75, 0.74, and 0.79, respectively. Using a cutoff value of 80% for FEF₅₀%pred achieved a sensitivity of 56.9% and specificity of 81.4%. A cutoff value of 74% for FEF₇₅%pred resulted in a sensitivity of 67.3% and specificity of 69.2%. Finally, using a cutoff value of 84% for MMEF%pred achieved a sensitivity of 67.9% and specificity of 77.2%. Conclusion: In the presence of normal ventilatory function, utilizing FEF₅₀<80% predicted or MMEF<84% predicted can accurately serve as criteria for identifying small airway dysfunction in children with controlled asthma.