Increased plasma levels of IL-6 in TRD patients compared HC. Decreased plasma levels of KYNA and significantly increased QUIN/KYNA ratio in TRDPlasma levels of tryptophan, kynurenine, and QUIN did not differ between patients and controls. There was a significant inverse correlation between symptom severity and kynurenine levels at baseline. Here, IL6 is linked to treatment resistant depression.