Analysis of complete blood counts and derived inflammatory indices with non-suicidal self-injury and suicide attempt in adolescents with major depressive disorder

Background Non-suicidal self-injury (NSSI) and suicide attempt (SA) are highly comorbid in adolescents with major depressive disorder (MDD), yet their distinct biological correlates are unclear. This study investigated the potential of complete blood count (CBC) parameters and derived inflammatory markers to differentiate clinical subtypes of adolescent MDD with NSSI and/or SA. Methods A retrospective cross-sectional study was conducted involving 618 adolescent inpatients with MDD. The participants were categorized into four clinical subgroups: pure MDD group (without NSSI and SA, n=382), NSSI-only group (n=131), SA-only group (n=41), and NSSI+SA comorbid group (n=64). CBC parameters and derived inflammatory indices, including the neutrophil-to-lymphocyte ratio (NLR), monocyte-to-lymphocyte ratio (MLR), platelet-to-lymphocyte ratio (PLR), and systemic immune-inflammation index (SII), were compared across groups. Statistical analyses included Kruskal-Wallis H tests with post-hoc comparisons, multivariate logistic regression, and receiver operating characteristic (ROC) curve analysis. Results The NSSI+SA comorbid group exhibited a robust inflammatory signature, with significantly elevated neutrophils, NLR, MLR, PLR, and SII, alongside reduced lymphocytes, compared to all the other subgroups (all p < 0.008). NLR was identified as a strong independent risk factor for the NSSI+SA comorbid state (odds ratios [ORs] ranging from 2.84 to 3.69). The NSSI-only group specifically exhibited elevated platelet crit (PCT) compared to the pure MDD group ( p = 0.004). In contrast, the SA-only group showed no distinct inflammatory profile compared to the pure MDD or NSSI-only groups. Conclusion NSSI and SA behaviors in adolescents with MDD are associated with distinct inflammatory biological bases. NSSI+SA comorbidity is linked to robust activation of the innate immune response, with elevated levels of NLR, MLR, PLR, and SII serving as potential auxiliary biomarkers for its identification.