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Dong Kyu Oh,1 Seunghee Baek,2 Sei Won Lee,1 Jae Seung Lee,1 Sang-Do Lee,1 Yeon-Mok Oh1 1Department of Pulmonary and Critical Care Medicine, Clinical Research Center for Chronic Obstructive Airway Diseases, Asan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea; 2Department of Clinical Epidemiology and Biostatistics, Asan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea Background and objective: Despite the ongoing intense debate on the definition of airflow limitation by spirometry in the elderly population, there have only been few studies comparing the fixed ratio and the Z-score of forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) in terms of long-term mortalities. In this study, we aimed to identify the proper method for accurately defining the airflow limitation in terms of long-term mortality prediction in the elderly population. Methods: Data were collected from the Third National Health and Nutrition Examination Survey in the US. Non-Hispanic Caucasians aged 65–80 years were included. The receiver operating characteristic (ROC) curves of both methods were plotted and compared for 10-year all-cause, respiratory, and COPD mortalities. Results: Of 1,331 subjects, the mean age was 71.7 years and 805 (60.5%) were males. For the 10-year all-cause mortality, the area under the curve (AUC) of the fixed ratio was significantly greater than that of the Z-score of FEV1/FVC, but both showed poor prediction performance (0.633 vs 0.616, p<0.001). For the 10-year respiratory and COPD mortalities, both the fixed ratio and the Z-score of FEV1/FVC showed comparable prediction performance with greater AUCs (0.784 vs 0.778, p=0.160, and 0.896 vs 0.896, p=0.971, respectively). Interestingly, the conventional cutoff of 0.7 in the fixed ratio was consistently higher than the optimal for the 10-year all-cause, respiratory, and COPD mortalities (0.70 vs 0.69, 0.62, and 0.61, respectively), whereas that of -1.64 in the Z-score of FEV1/FVC was consistently lower than the optimal cutoff (-1.64 vs -1.31, -1.47, and -1.41, respectively). Conclusion: In the elderly population, both the fixed ratio and the Z-score of FEV1/FVC showed comparable prediction performance for the 10-year respiratory and COPD mortalities. However, the conventional cutoff of neither 0.70 in the fixed ratio nor -1.64 in the Z-score of FEV1/FVC was optimal for predicting the long-term mortalities. Keywords: airway obstruction, chronic obstructive pulmonary disease, mortality, pulmonary disease, ROC curve, spirometry