Objective To investigate the serum C5b-9 level in adult patients with idiopathic membranous nephropathy (IMN) and its association with clinical indicators, and to evaluate its auxiliary diagnostic value. Methods A total of 100 adult patients with renal biopsy-diagnosed IMN (IMN group) at the First Affiliated Hospital of Guangxi Medical University between July 2020 and November 2022 were enrolled, together with 100 healthy individuals undergoing physical examinations (healthy control group) at the same hospital during the same period. Baseline clinical data of all research subjects and pathological data of IMN patients were collected. According to the Kidney Disease: Improving Global Outcomes (KDIGO) guidelines, the risk of renal function progression was assessed, and IMN patients were divided into a low-risk group or a medium-to-high-risk group; based on 24-hour urine protein quantification, IMN patients were categorized into a non-massive proteinuria group (≤3.5 g/24 h) or a massive proteinuria group (>3.5 g/24 h). Serum C5b-9 levels were compared between groups. Pearson correlation analysis and multiple linear regression analysis were used to explore the association between serum C5b-9 level and clinical indicator, and logistic regression analysis was employed to investigate factors influencing massive proteinuria in IMN patients. The receiver operating characteristic (ROC) curve was plotted to evaluate the diagnostic efficacy of serum C5b-9 level for IMN. Results The serum C5b-9 level was higher in the IMN group than in the healthy control group ([174.57±33.19] ng/mL vs. [151.18±15.33] ng/mL, P<0.05). Among IMN patients. the medium-to-high-risk group had a higher serum C5b-9 level than the low-risk group ([181.56±31.06] ng/mL vs. [149.81±31.05] ng/mL, P<0.05); the massive proteinuria group had a higher level than the non-massive proteinuria group ([182.41±31.51] ng/mL vs. [161.51±32.41] ng/mL, P<0.05); however, there was no statistically significant difference in serum C5b-9 level among groups with different renal pathological features (all P>0.05). Pearson correlation analysis showed that serum C5b-9 level was positively correlated with 24-hour urine protein quantification, and negatively correlated with serum immunoglobulin G and albumin levels (all P<0.05). Multiple linear regression analysis indicated that 24-hour urine protein quantification was an independent influencing factor for serum C5b-9 level (P<0.05). Multivariate binary logistic regression analysis showed that the serum C5b-9 level was an independent risk factor for massive proteinuria in IMN patients (OR=1.014, 95% CI: 1.000-1.029, P<0.05). ROC curve analysis showed that the area under the curve for serum C5b-9 level in diagnosing IMN was 0.717, with an optimal cut-off value of 164.91 ng/mL, corresponding to a sensitivity of 75.02% and a specificity of 60.73%. Conclusion Serum C5b-9 level is elevated in adult IMN patients, and the elevation is closely associated with more severe clinical phenotypes (medium-to-high risk of renal function progression and massive proteinuria). Serum C5b-9 level is an independent risk factor for massive proteinuria in adult IMN patients and is positively correlated with 24-hour urine protein quantification. Furthermore, serum C5b-9 level has certain auxiliary diagnostic value for IMN.

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