The accuracy of FEF25-75/ FVC for primary classification of the pulmonary function test

Introduction: The forced expiratory flow at 25 and 75% of the pulmonary volume/forced vital capacity ratio (FEF25-75/FVC) as a spirometry parameter has been successful in the early diagnosis of chronic obstructive pulmonary disease (COPD) and the methacholine challenge test for assessing airway responsiveness.To determine the accuracy of FEF25-75/FVC for the classification of spirometry lung disease. Materials and Methods: Eighty subjects with clinical diagnosis of COPD and idiopathic pulmonary fibrosis (IPF) were entered into this case-control study. Forty normal volunteers in the control group with a PC20 of more than 8 mg/dl were also enrolled in this study. Spirometry, lung volumes, and diffusing capacity (DLCO) were measured for all the subjects by the body plethysmograph. Final diagnosis of COPD and IPF was confirmed according to patient's history, pulmonary function test, computed tomography of the lungs, and histopathology (in IPF subjects). The FEF25-75/FVC ratio was determined in each group, and test accuracy was compared with lung volumes and DLCO as the gold standard. Results: FEF25-75/FVC was able to divide the subjects into four categories and its agreement with the clinical diagnosis (kappa= 0.486) was more than the ratio of forced expiratory volume in one second per forced vital capacity (FEV1/FVC) and residual volume (RV). Accuracy assessment showed that FEF25-75/FVC had the highest likelihood ratio (133) followed by FEV1/FVC. Mid-expiratory flow parameters including FEF25-75 and FEF25-75/FVC displayed the highest sensitivity, positive predicted value, negative predicted value, and accuracy. Conclusion: FEF25-75/FVC is helpful in diagnosing difficult cases such as mixed-type spirometry or spirometry results that are not matched with clinical findings and require lung volume measurement.