Supplementary Material for: An increase in C-reactive protein levels during antidepressant treatment as a candidate marker for treatment non-response in major depressive disorder

Introduction One-third of patients with major depressive disorder (MDD) exhibit low-grade inflammation as reflected by C-reactive protein (CRP) concentrations > 3 mg/L. We explored whether CRP changes from baseline to week one of antidepressant treatment (ΔCRP) can serve as a marker of treatment response. Methods CRP serum levels were measured at baseline and after the first week of treatment in 33 MDD patients and correlated with patients’ Hamilton depression scale (HAM-D), while adjusting for age, gender and body mass index. We assessed antidepressant responses at weeks one and four of treatment as a > 25% and > 50% HAM-D score reduction (ΔHAM-D) compared to baseline, respectively. We compared baseline and week-one CRP levels with the paired t-test within responders and non-responders separately and ΔCRP between the groups with the ANCOVA. Results Higher ΔCRP correlated with lower week-four ΔHAM-D scores (r = -0.5, p = 0.006). Non-responders showed higher ΔCRP – but not baseline and week-one CRP – than responders (p = 0.018, Cohen’s d = 1.1). A ΔCRP increase was observed in 13/16 (81%) non-responders and 7/17 (41%) responders (Fisher’s exact test’s p = 0.03). A ΔCRP increase combined with a week-one non-response was observed in 13/16 (81%) non-responders and 1/17 (6%) responders (p < 0.0001). Conclusions Rather ΔCRP at week one than baseline CRP might be indicative of treatment response at week four, especially if combined with week-one ΔHAM-D. In the future, ΔCRP could be introduced into psychiatric practice to guide treatment plans.