Serum levels of pro-inflammatory cytokine TNF-α (pg/ml, median) in patients with cLBP and HC during 6 months' follow-up. The serum level of TNF-α was measured using the Bio-Plex cytokine assay (Bio-Lab, Munich, Germany). The y-axis shows the median values of TNF-α serum levels (pg/ml). The Mann-Whitney-Test showed that serum levels of TNF-α of cLBP patients both with and without depression (DE) were significantly higher than those of healthy controls at day 0, but not at days 10, 21 and 180. **: P < 0.01, differences between patient groups and healthy controls; ###: P < 0.001, ##: P < 0.01, #: P < 0.05, decrease between T0 and T1 or T0 and T3. cLBP, chronic low back pain; HC, healthy controls.