Objective To evaluate the relationship between ceftazidime-avibactam(CAZ-AVI)blood concentrations and efficacy in critically ill patients and to investigate the factors influencing blood levels.Methods The CAZ-AVI trough concentrations(Cmin)were detected in 29 patients who received CAZ-AVI treatment for at least 48 hours.The clinical materials of the patients were collected together for retrospective analysis.Results The Cmin of ceftazidime(CAZ)and avibactam(AVI)were(50.95±5.17)and(7.52±0.96)mg·L-1 in the effective group and(31.16±7.03)and(5.37±1.32)mg·L-1 in the ineffective group,respectively.The Cmin of CAZ in the effective group was significantly higher than in the ineffective group(P<0.05),and there was no significant difference in AVI Cmin between the two groups(P>0.05).Spearman's correlation analysis showed that CAZ Cmin was positively correlated with clinical efficacy(P<0.05),and no correlation between AVI Cmin and clinical efficacy(P>0.05).The optimal CAZ Cmin threshold was 24.59 mg·L-1.Multiple linear regression analysis showed that age and creatinine clearance was significantly correlated with the Cmin of CAZ,and creatinine clearance was significantly correlated with AVI Cmin(P<0.05).Conclusions The Cmin of CAZ correlates with efficacy,and it may be more beneficial for clinical treatment to keep the concentration of CAZ-AVI always greater than the minimum inhibitory concentration during the dosing interval.The creatinine clearance should be fully considered when optimizing CAZ-AVI dosage in critically ill patients.