Bidirectional Associations Between Inflammatory Biomarkers and Depressive Symptoms in Adolescents: Potential Causal Relationships.

There are inconsistent findings in the literature about the directionality and magnitude of the association between inflammation and depressive symptoms. This analysis separates predictors into between-person and within-person components to gain greater clarity about this relationship. Blood samples were collected and depressive symptoms assessed in 140 adolescents (54% female, 59% Black, Mage = 16.1 years) with at least three blood draws and a total of 394 follow-up observations. Multi-level modeling indicated that the within-person effect of tumor necrosis factor alpha (TNF-α) predicted change in total depressive symptoms, suggesting a potential causal relationship. There were no significant within-person effects of total depressive symptoms on change in biomarkers. Exploratory analyses examined associations between inflammatory biomarkers and subsets of depressive symptoms. These findings inform modeling decisions that may explain inconsistencies in the extant literature as well as suggest potential causal relationships between certain proteins with significant within-person effects on depressive symptoms, and vice-versa.