Plasma Cathepsin S and Cathepsin S/Cystatin C Ratios Are Potential Biomarkers for COPD.

. This study aimed to examine whether plasma levels of cathepsin S or its inhibitor, cystatin C, may serve as biomarkers for COPD. . We measured anthropometrics and performed pulmonary function tests and chest CT scans on 94 patients with COPD and 31 subjects with productive cough but no airflow obstruction ("at risk"; AR). In these subjects and in 52 healthy nonsmokers (NS) and 66 healthy smokers (HS) we measured plasma concentrations of cathepsin S and cystatin C using an ELISA. Data were analyzed using simple and logistic regression and receiver operating characteristic analyses. . Cathepsin S and cystatin C plasma levels were significantly higher in the COPD and AR groups than in the NS and HS groups ( < 0.01). Among the COPD patients and AR subjects, plasma cathepsin S levels and cathepsin S/cystatin C ratios, but not cystatin C levels, were negatively related to severe airflow limitation (% FEV predicted < 50%; = 0.005) and severe emphysema as assessed by low attenuation area (LAA) score on chest CT scans (LAA ≥ 8.0; = 0.001). . Plasma cathepsin S and cathepsin S/cystatin C ratios may serve as potential biomarkers for COPD.