It is important to realize, however, that restrictive lung disease cannot be diagnosed by spirometry alone. With severe airways obstruction the lung volume remaining in the thorax after expiration (the functional residual capacity) may be increased to such a degree that it limits inspiratory capacity and, hence, reduces vital capacity. A reduced FEV1 and FVC are therefore only suggestive of true restrictive disease, but it is necessary to measure lung volumes to accurately diagnose restrictive physiology.