CXCL8 and major depressive disorder: However, there are several obstacles to implementing IL-8 as a clinical biomarker, including pre-analytical variability (sample type), assay heterogeneity used to determine IL-8 level (ELISA vs. multiplex platforms yield different results), influence of comorbidities (obesity, infections, autoimmune disorders may affect IL-8 independently of depression), or even sex and age effects, which require stratified analyses.