IL6 and depressive disorder: Overall previous CSF biomarker studies of patients with depression compared to healthy controls vary in sample size with markers like IL-8 and CSF total protein measured in large cohorts of patients (n = 104 [28] and n = 90 [29], respectively), while other important markers like IL-6 and IgG index are measured only in rather small cohorts of patients (n = 30 [30] and n = 29 [19], respectively).