This study examined serum Claudin-5 alongside systemic inflammatory indices-including the neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), monocyte-to-lymphocyte ratio (MLR), the inflammation score (INFLA), and suicidal ideation in antidepressant-naive or medication-free (for at least three months) adult MDD patients.<h4>Method</h4>73 antidepressant-naive or drug-free MDD patients and 74 age- and sex-matched healthy controls were enrolled. The gene discussed is CLDN5; the disease is major depressive disorder.