The systemic immune-inflammation index (SII) as a biomarker for depression in a community sample of adolescents

Depression is associated with increased levels of pro-inflammatory biomarkers in children and adolescents. As research to date has primarily focused on inflammatory cytokines, the potential role of white blood cells (WBCs) and platelets in the inflammatory response is not well understood. This study examines the association of blood cell based inflammatory indices, including the systemic immune-inflammation index (SII), and depressive symptoms in participants in the Adolescent Brain Cognitive Development Study.

Adolescents were recruited from community settings and completed self-report measures of depression symptoms and semi-structured psychiatric interview to determine depression diagnosis. Participants provided blood samples to obtain absolute counts of neutrophil, lymphocyte, monocyte, and platelet levels for calculation of inflammatory indices. The association between depression and inflammatory markers was examined while accounting for participant age, sex, ethnicity, comorbid psychiatric disorder, parental education and annual household income.

Of 858 participants (mean age: 12.4 ± 1.1 years; 45 % female), 101 received a diagnosis of a depressive disorder. Greater depressive symptoms were significantly associated with higher neutrophil and platelet levels (β = 0.013 and β = 0.018, respectively) and higher SII (β = 0.012), after adjusting for covariates. Diagnosis of depression was not associated with WBC levels or indices.

In this community-based sample of adolescents, greater depressive symptoms were associated with elevated SII and individual white blood cell levels. Future studies using larger, longitudinal clinical samples are needed to confirm the potential role of the SII in adolescent depression, and the involvement of inflammation in early-onset depression.