CLDN5 and major depressive disorder: ROC analysis for serum Claudin-5 indicated fair accuracy in distinguishing MDD from controls (AUC = 0.737) but limited value for predicting suicidal ideation (AUC = 0.628).<h4>Conclusion</h4>Reduced serum Claudin-5 in untreated MDD may indicate relatively stable endothelial alterations rather than acute, state-dependent changes.