Fig. 4.

Association of the PHQ-9 sum score to different definitions of mental disorders. This figure illustrates the mean PHQ-9 sum score (black bar), the mean PHQ-9 sum score adjusted for the effect of COPD severity (dark grey bar) and the mean PHQ-9 sum score adjusted for the effect of comorbidities (light grey bar) for all patients (n = 2096), for patients with the reported diagnosis of mental disorders (n = 464) and for patients with the intake of anti-depressants (n = 233). The figure shows that, independent of the subpopulation chosen for analysis, the procedure to take into account COPD characteristics via symptoms/exacerbation risk or comorbidities leads to about the same reduction of the observed PHQ-9 sum score. Moreover it illustrates that both, before and after adjustment, patients with antidepressant therapy showed scores between 6.8 and 9.2, which were larger than those reported in the literature in patients without COPD. This suggests that the adjustment did not fully account for all COPD characteristics, considering the fact that the GOLD group A and the absence of comorbidities were the reference