The Concave Shape of the Forced Expiratory Flow-Volume Curve in 3 Seconds Is a Practical Surrogate of FEV₁/FVC for the Diagnosis of Airway Limitation in Inadequate Spirometry

BACKGROUND: Spirometry is important for the differential diagnosis of dyspnea. However, some patients cannot exhale for ≥6 s to achieve the American Thoracic Society/European Respiratory Society criteria. The aim of this study was to demonstrate the reliability of a new parameter that quantifies the degree of concavity in the first 3 s to define airway limitation as a surrogate for the FEV₁/FVC.

METHODS: Four hundred spirometry test results were selected through complete random sampling. The new parameter, termed the AUC₃/AT₃, was calculated as the area under the descending limb of the expiratory flow-volume curve before the end of the first 3 s (AUC₃) divided by the area of the triangle before the end of the first 3 s (AT₃). The AUC₃/AT₃ was compared with the FEV₁/FVC using Pearson's correlation analysis. The level of agreement between the AUC₃/AT₃ and the FEV₁/FVC in the detection of airway obstruction was analyzed using the kappa statistic. We also compared the diagnostic accuracy of the new index with that of the FEV₁/forced expiratory volume in the first 3 s (FEV₃).

RESULTS: There was a strong correlation (r = 0.88, P < .001) between the AUC₃/AT₃ and the FEV₁/FVC. There was also strong agreement between the AUC₃/AT₃ and the FEV₁/FVC in the detection of obstruction with kappa indices of 0.72 (Global Initiative for Chronic Obstructive Lung Disease [GOLD] criterion) and 0.67 (lower limit of normal criterion), and these values were greater than those obtained for the FEV₁/FEV₃. The AUC₃/AT₃ also exhibited acceptable sensitivity, specificity, positive predictive value, and negative predictive value. The diagnostic accuracies of the AUC₃/AT₃ were 86.3% (GOLD criterion) and 83.8% (lower limit of normal criterion), which were greater than the 76.0 and 74.0% obtained for the FEV₁/FEV₃, respectively.

CONCLUSIONS: The AUC₃/AT₃ can be utilized as a surrogate parameter for the FEV₁/FVC when patients cannot complete a 6-s expiratory effort. Additionally, the performance of this index is better than that of the FEV₁/FEV₃ in the identification of airway limitations.