Clinical Implications of Having Reduced Mid Forced Expiratory Flow Rates (FEF25-75), Independently of FEV1, in Adult Patients with Asthma.

INTRODUCTION FEF25-75 is one of the standard results provided in spirometry reports; however, in adult asthmatics there is limited information on how this physiological measure relates to clinical or biological outcomes independently of the FEV1 or the FEV1/FVC ratio. PURPOSE To determine the association between Hankinson's percent-predicted FEF25-75 (FEF25-75%) levels with changes in healthcare utilization, respiratory symptom frequency, and biomarkers of distal airway inflammation. METHODS In participants enrolled in the Severe Asthma Research Program 1-2, we compared outcomes across FEF25-75% quartiles. Multivariable analyses were done to avoid confounding by demographic characteristics, FEV1, and the FEV1/FVC ratio. In a sensitivity analysis, we also compared outcomes across participants with FEF25-75% below the lower limit of normal (LLN) and FEV1/FVC above LLN. RESULTS Subjects in the lowest FEF25-75% quartile had greater rates of healthcare utilization and higher exhaled nitric oxide and sputum eosinophils. In multivariable analysis, being in the lowest FEF25-75% quartile remained significantly associated with nocturnal symptoms (OR 3.0 [95%CI 1.3-6.9]), persistent symptoms (OR 3.3 [95%CI 1-11], ICU admission for asthma (3.7 [1.3-10.8]) and blood eosinophil % (0.18 [0.07, 0.29]). In the sensitivity analysis, those with FEF25-75% <LLN had significantly more nocturnal and persistent symptoms, emergency room visits, higher serum eosinophil levels and increased methacholine responsiveness. CONCLUSIONS After controlling for demographic variables, FEV1 and FEV1/FVC, a reduced FEF25-75% is independently associated with previous ICU admission, persistent symptoms, nocturnal symptoms, blood eosinophilia and bronchial hyperreactivity. This suggests that in some asthmatics, a reduced FEF25-75% is an independent biomarker for more severe asthma.