Because the process of aging affects lung volumes, the use of this fixed ratio may result in the overdiagnosis of airflow obstruction in elderly persons, especially in those with mild disease. Therefore, the current Global Initiative for Chronic Obstructive Lung Disease guidelines7 advise that using a lower limit of normal values for FEV1/FVC ratio, which is based on a normal distribution and classifies the bottom 5% of the healthy population as abnormal, is one way to minimize the potential misclassification. If a lower limit is used for FEV6, it should be applied to FVC too. We think that no remarkable difference would be seen in the results while evaluating the FEV1/FEV6. Also, the simplicity of using FEV6 in place of FVC would be sacrificed if a lower limit for FEV6 is utilized. However, reference equations using post-bronchodilator therapy FEV1 and longitudinal studies to validate the use of the lower limit of normal are urgently needed. The only such equations currently available are those from the National Health and Nutrition Examination Study III study.