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Çağlayan performed the patient recruitment and clinical follow-up. All collaborators contributed to the clinical management of the COVID-19 patients. Y. Peker, Y. Celik and A. Baygül performed the statistical analysis. Y. Peker prepared the manuscript, and drafted the article. All authors interpreted the data. Y. Peker takes full responsibility for the work as a whole, including the study design, access to data, and the decision to submit and publish the manuscript. All authors approved this manuscript in its final form. Data Sharing: Data collected for the study, including de-identified individual participant data will be made available to others within 6 months after the publication of this article, as will additional related documents (study protocol, statistical analysis plan, and informed consent form), for academic purposes (e.g., meta-analyses), upon request to the corresponding author

INTRODUCTION: By examining the relationship between changes in maximum mild-expiratory flow( MMEF) and specific airway conductance(sGaw), parameters with the change in FEV1 when evaluating the spirometer test and the bronchodilator response, we investigated their diagnostic contribution to the bronchodilator response in those with partial responses to FEV1. METHODS: The retrospective study sample consists of data from 112 patients between Jun 1, 2019, and Feb 1, 2020 who applied to the pulmonary function laboratory with a pre-diagnosis of bronchial hyperreactivity as well as body plethysmography test performed together with the reversibility test. RESULTS: MMEF% and ▲sGaw were linearly correlated with ▲FEV1 (respectively r = 0.752; p <0.001, r = 0.611; p <0.001). While there was a significant difference between ▲MMEF% and ▲sGaw between reversible, partially reversible, and irreversible groups (P <0.001), there was no significant difference in ▲sGaw between partial reversible and reversible groups in post-hoc comparisons (P> 0.05). In the binary logistic regression model created between the partially reversible and reversible groups, demographic characteristics, MMEF% and ▲sGaw variables, ▲MMEF was an independent predictor [OR: 1.132; 95% CI (1.036-1.238), p = 0.006]. The 24% threshold for absolute reversibility or partial reversibility calculated with MMEF% was significant, indicating significance at 86.2% sensitivity and 80.8% specificity (AUC: 0.811, 95% CI: 0.686-0.936; p <0.001). This value we found defined 81% of the partially reversible group as reversible. DISCUSSION AND CONCLUSION: We observed that ▲sGaw alone didn' have a determinant contribution for determining reversibility in bronchodilator response, which showed partial reversibility with respect to FEV1 change. We believe that ▲MMEF% can be an independent predictor between these two groups and the calculated threshold value of 24% can be used as a criterion for determining the reversibility in cases where FEV1 is not determinant.