Several previous studies have investigated the correlation between CT lung density and physiology with varying results.15 To our knowledge only 6 studies have examined the correlation between PD15 and FEV1 in percent of predicted and they found correlations ranging from 0.09-0.62.44-48 This is likely due to the fact that all of these studies were based on more selected populations with a limited range of airflow limitation and lung densities which may explain poor correlations. We used simple Pearson correlation on both the baseline values and the slopes to compare the correlation between baseline values with the correlation between slopes in this large cohort, and refrained from using multiple correlations and more sophisticated statistical models and adjustments as these did not seem to add value given the aim of our study.