Abstract:Objective To investigate the associations of serum complement component 3a（C3a）and soluble terminalcomplement complex C5b-9（sC5b-9）levels with the severity of sepsis and 28-day clinical prognosis among patients，and to provide new biomarker references for early risk stratification and prognostic assessment of sepsis. Methods A total of 209 sepsis patients admitted to Kunshan First People's Hospital Affiliated to Jiangsu University from March 2022 to September 2024 were retrospectively included as the study group. They were categorized into shock group（n=129）and non-shock group（n=80）according to the severity of the disease. Based on the survival status of the patients within 28 days after admission，they were divided into survival group（n=137）and death group（n=72）. Another 56 physically healthy individuals were selected as the control group during the same period. The clinical data of all subjects were collected. Serum C3a and sC5b-9 levels were compared between the groups. The associations of serum C3a and sC5b-9 levels with the severity and prognosis of sepsis patients were analyzed by Spearman correlation analysis. The value of the area under the curve（AUC）of the receiver operating characteristics（ROC）was used to assess the effects of serum C3a and sC5b-9 on predicting poor prognosis in patients with sepsis. Independent influencing factors on poor prognosis in patients with sepsis were analyzed using univariate and multivariate logistic regression. Results C3a and sC5b-9 levels were significantly higher in the non-shock group［（169.25±21.47）ng/mL，（325.69±25.36）ng/mL］and shock group［（198.74± 19.86）ng/mL，（356.98± 36.21）ng/mL］compared with those in the control group［（98.25± 19.25）ng/mL，（89.36±12.14）ng/mL，P<0.05］. C3a and sC5b-9 levels were significantly higher in the shock group compared with those in the non-shock group（P<0.05）. C3a and sC5b-9 levels were significantly higher in the death group compared with those in the survival group（P<0.05）. Spearman correlation analysis showed that serum C3a and sC5b-9 levels were positively correlated with the severity of sepsis（r=0.802，0.744，P<0.05）and the poor prognosis（r=0.507，0.602，P<0.05）. The ROC analysis results showed that the AUC of the combination of C3a and sC5b-9 in predicting the poor prognosis of sepsis patients was 0.910，with a sensitivity of 80.56% and a specificity of 87.59%. The results of multivariate logistic regression analysis showed that C3a，sC5b-9，white blood cell count（WBC），serum amyloid A（SAA），total bilirubin（TBIL），and Sequential Organ Failure Assessment（SOFA）score were independent risk factors for poor prognosis in patients with sepsis （P<0.05）. Conclusion Serum C3a and sC5b - 9 levels are significantly correlated with the severity and poor prognosis of sepsis patients，and the combined detection of these two has a high predictive value for the poor prognosis of sepsis patients，which can be used as an effective biomarker for the clinical assessment of the severity and prognosis of sepsis.