Objective To explore the expression and significance of cathepsin K (CatheK) and fluorescent oxidation product_400 (FlOP_400) in peripheral blood of patients with osteoporosis.

Methods A total of 102 patients with osteoporosis in the Department of Endocrinology and Immunology of Yulin Hospital of the First Affiliated Hospital of Xi'an Jiaotong University from February 2020 to December 2023 were enrolled as osteoporosis group, and 62 healthy volunteers selected as control group. Levels of CatheK, FlOP_400, serum bone metabolism markers, and bone mineral density were detected; the Pearson correlation analysis was used to explore the correlations of CatheK, FlOP_400, bone metabolism markers, with bone mineral density; risk factors associated with osteoporosis were analyzed, and receiver operating characteristic (ROC) curve was plotted to assess the diagnostic values of CatheK and FlOP_400 for osteoporosis.

Results The levels of CatheK, FlOP_400, β-crosslaps (β-CTx), and receptor activator of nuclear factor-κB ligand (RANKL) in the peripheral blood of the osteoporosis group were significantly higher than those in the control group, while the levels of serum procollagen type Ⅰ N-terminal propeptide (PINP), osteopontin (OPN), osteocalcin (OC), osteoprotegerin (OPG), and bone mineral density were significantly lower than those in the control group (P < 0.05). The levels of CatheK and FlOP_400 in peripheral blood of patients with osteoporosis were significantly positively correlated with β-CTx and RANKL (P < 0.05), and negatively correlated with BMD, PINP, OPN, OC, and OPG (P < 0.05). History of smoking, history of alcohol consumption, lack of exercise, high CatheK level, and high FlOP_400 level were risk factors for osteoporosis (P < 0.05). The ROC curve showed that the areas under the curve for the diagnosis of osteoporosis by CatheK and FlOP_400 were 0.799 and 0.780 respectively, and the area under the curve for combined diagnosis was 0.889, which was significantly higher than that for individual diagnosis (P < 0.05).

Conclusion The levels of CatheK and FlOP_400 in peripheral blood of patients with osteoporosis are significantly increased and associated with decreased bone mineral density and abnormal bone metabolism. Combined detection of CatheK and FlOP_400 has high value in the diagnosis of osteoporosis.