MMEF25-75 may predict significant BDR and future risk of exacerbations in asthmatic children with normal baseline FEV1.

(1) Background: Several recent studies on the clinical value of spirometry indexes demonstrated high sensitivity of FEF_25-75 as a marker of bronchial obstruction in asthmatics with normal baseline spirometry. Our study aims to evaluate the clinical value of maximal mid-expiratory flow in children with asthma. (2) Methods: For two years, 257 children were included - 211 with asthma and 46 healthy controls. Pre- and post-bronchodilator spirometry, atopic status determination and asthma control assessment were performed. (3) Results: The small airway obstruction (SAO) group (FEV₁≥80%, ММEF_25/75<65%) demonstrated significantly lower values for FEV₁, FEV₁/FVC, PEFR, МMMF_25/75 and a significant higher bronchodilator response (BDR, ΔFEV₁% init. ≥12%) compared to normal baseline spirometry group (FEV₁>80%, MMEF_25/75≥65%) (Р<0.0001). In addition, we found a statistically significant difference in FEF_25-75/FVC median between asthmatics and healthy controls (Р<0.0001) regardless of the FEV₁ value. Children with SAO have a 2.338-fold higher risk of poor asthma outcome (OR 95% CI [1.077-5.294]) and a 6.171-fold (OR 95% CI [2.523-15.096]) greater probability of demonstrating positive BDR, compared to children with normal baseline spirometry. MMEF_25/75 was found to be a good predictor for positive BDR with AUC 0.843 (CI 0.781-0.845) and a best cut-off value of 58.1% (77.8% sensitivity and 78.8% specificity). (4) Conclusion: Our results confirmed that a small but substantial group of asthmatic children with normal baseline FEV₁ and low MMEF_25-75 are at higher risk for poor asthma outcomes.