Objective:To explore the association of frailty and serum C-terminal agrin fragment(CAF)with the prognosis of elderly patients with acute coronary syndrome(ACS).Methods:In this prospective cohort study, clinical data of 207 older patients with ACS between January 2020 and May 2022 were collected.Serum samples were obtained within 24 hours after enrollment to detect CAF levels.Meanwhile, the thrombolysis in myocardial infarction(TIMI)and frailty screening questionnaire(FSQ)scores were assessed on admission.Patients were followed up for major adverse cardiovascular and cerebrovascular events(MACCE)for 90 days.Multivariate logistic regression was used to analyze the influencing factors of MACCE.The receiver operating characteristic(ROC)curve was performed to evaluate the predictive ability of the FSQ score, serum CAF and their combination for MACCE.According to 90-day mortality, patients were divided into a survival group(n=176)and a death group(n=31). The Cox proportional hazards regression model was used for survival analysis.Results:The FSQ score( Z=4.412, P<0.001)and serum CAF( Z=6.702, P<0.001)in the MACCE group were higher than those in the non-MACCE group.Logistic regression analysis showed that after adjusting for age, sex, TIMI score and complete revascularization, frailty defined by FSQ( OR=1.714; 95% CI: 1.059-2.775; P=0.028)and high serum CAF( OR=1.230; 95% CI: 1.122-1.350; P<0.05)were independent risk factors for MACCE.The area under the ROC curve(AUC)of the FSQ score for predicting MACCE was 0.797(95% CI: 0.735-0.850; P<0.001), the predictive cut-off point was an FSQ score >2, and the Youden index(YI)was 0.419, yielding a sensitivity of 0.708 and a specificity of 0.711.In addition, the AUC of serum CAF for predicting MACCE was 0.766(95% CI: 0.701-0.822; P<0.001), the predictive cut-off point was >6.01 μg/L, and YI was 0.460, yielding a sensitivity of 0.750 and a specificity of 0.710.The predictive ability of FSQ combined with CAF for MACCE was higher than FSQ( Z=2.294, P=0.022)or CAF( Z=2.545, P=0.011)alone.Cox regression analysis showed that frailty defined by FSQ( HR=3.487; 95% CI: 1.329-9.153; P=0.011)was independently associated with all-cause mortality within 90 days after ACS. Conclusions:Frailty assessment and serum CAF detection can improve the risk stratification of elderly patients with ACS.