DLCO versus DLCO/VA as predictors of pulmonary gas exchange

Abstract

The diffusing capacity of the lung is usually reported as both the diffusing capacity (DLCO) and the diffusing capacity divided by the alveolar volume (DLCO/VA). However, it is unclear which measure to use when interpreting pulmonary gas exchange. We therefore conducted this study to determine whether the DLCO or the DLCO/VA is a better predictor of oxygen desaturation with exercise. We retrospectively analyzed the pulmonary function records of all patients who had measurement of their diffusing capacity and 6-min walk oximetry in our university pulmonary function laboratory over a 2-year period. There were data available on 97 patients, most of who had interstitial lung disease and/or lung volume restriction. The median DLCO was 51% predicted and the median DLCO/VA was 64% predicted. The prevalence of exercise desaturation was 43%. The overall sensitivity and specificity as determined by the area under the receiver operator characteristic (ROC) curve was higher for DLCO than DLCO/VA, with an optimal cut-off of normal of 55% predicted. The positive predictive values were equally low for both measures, ranging from 50% to 70%. After adjustment for VA, there were no differences between the ROC curves or predictive values for DLCO and DLCO/VA. After adjusting for VA, neither the DLCO nor the DLCO/VA was better at predicting oxygen desaturation with exercise. The optimal cut-off of normal was 55% predicted.