In general, this is my approach to interpreting a pulmonary function test: First, I look at the FEV1/FVC ratio. If this is below 75%, an obstructive ventilatory defect is present. If the ratio is above 75%, are the FVC and FEV1 low? If so, a restrictive ventilatory defect is likely and I next look at the lung volume section.
A low Vital Capacity confirms a restrictive ventilatory defect. Some regard the Total Lung Capacity as confirmatory, so I consider both. In general, in a restrictive ventilatory defect, the majority of lung volumes will be decreased. In an obstructive ventilatory defect is present, the Residual Volume will be high.
I also look at the shape of the flow volume loop to differentiate an obstructive ventilatory defect from a restrictive ventilatory defect. An obstructive ventilatory defect gives a distinctive pattern. A restrictive ventilatory defect looks like a normal flow volume loop, except that it is smaller.
As far as diffusion studies, a low DLCO can be seen in both emphysema and interstitial fibrosis. The DLCO can help differentiate the cause of a restrictive or obstructive ventilatory defect. In a restrictive ventilatory defect, interstitial fibrosis may cause a decrease in DLCO but chest wall deformities and obesity would not. In an obstructive ventilatory defect, emphysema may cause a decrease in DLCO but asthma would not. The DLCO/VA is an attempt to correct the diffusion for the actual lung volume being tested. A normal value should imply that the cause of the restriction is extrinsic to the lungs (such as a chest wall deformity or obesity) instead of intrinsic to the lungs (emphysema and interstitial fibrosis). However, a normal value can occur in any of these conditions.