for FVC and FEV1 that are over 80% of predicted are defined as within the normal
range. The FEV1/FVC ratio is expressed as a percentage, and a normal young individual is
able to forcibly expire at least 80% of his or her vital capacity in one second. A ratio
under 70% suggests underlying obstructive physiology. However, the FEV1/FVC ratio declines
as a normal sequelae of aging. Thus, at advanced ages, pathologic airways obstruction is
diagnosed based upon deviation from predicted FEV1/FVC values, with values below the 5th
percentile best selecting patients with obstructive defects.