IL6 and major depressive disorder: Elevated QA/KYNA ratios have been consistently reported in patients with MDD, bipolar depression, and chronic fatigue and are correlated with symptom severity, cognitive impairment, and the levels of circulating proinflammatory cytokines such as IFN-γ, IL-6, and TNF-α (see the next section for a more elaborate explanation) [24,34].