CST3 and chronic obstructive pulmonary disease: Restricted cubic spline analysis revealed a significant non-linear dose-response relationship (<i>P</i> = 0.023).<h4>Conclusion</h4>Serum cystatin C strongly correlates with AECOPD severity across multiple assessment scales and demonstrates good diagnostic accuracy, supporting its potential as a reliable biomarker for clinical severity evaluation in COPD exacerbations.