The author wonders whether an inappropriate density threshold limit was applied during our CT scan analysis and cites results published by Zompatori et al3 that suggest a 230-mL increase of measured air volume per 100 Hounsfield units (HU) of change in the higher limit of the density mask. First, we referenced tracheal air on each individual scan and used this value to identify tissue density on each section. Thus, we did not assume that any given mask density was uniformly accurate. Second, Zampatori et al3 studied 17 patients with airflow obstruction by comparing CT scan-derived volume estimates at masks densities of 200, 300, and 400 HU with volumes measured plethysmographically. In effect, they assumed that plethysmography was the gold standard and concluded that CT scan volume estimates obtained with a 200-HU density mask were most accurate (“The upper threshold was more correct…”) because they most closely corresponded with those measured by body box. Their CT scan-derived TLC estimates obtained with a 200-HU mask density averaged 873 mL less than those obtained by plethysmograph. This is precisely equivalent to the mean plethysmographic-to-CT scan volume difference we report in Table 2.