Abstract: Objective The purpose of this study is to investigate the distribution of pathogens, clinical characteristics and risk factors of polymicrobial bloodstream infections(PBSIs)in adult patients. Methods In a retrospctive matched case-control study design, all hospitalised adults with PBSIs from November 2018 to August 2021 were recruited. For each PBSI case, two patients with monomicrobial bloodstream infections(MBSIs)by pathogens and period were matched. All MBSIs cases matched composed the control group. The distribution of pathogens of the case group was analyzed. The clinical data of the two groups were compared. The multivariable analysis was performed using conditional logistic regression to determine the independent risk factors associated with PBSIs. Results Thirty-eight episodes of PBSI（2.9% of all adult bloodstream infections）and 76 pathogens were identified. The first two strains of fifty-four gram-negative bacill were escherichia coli and klebsiella pneumoniae. The majority of gram-positive bacteria were Enterococcus. Half of the pathogen combinations were the combination of gram-negative bacilli. Solid malignant tumor[P=0.016; odds ratio (OR)= 4.862]and biliary tract infections（P=0.032;OR=3.704）were independent risk factors for PBSIs. Patients with PBSIs had higher percentage of antibiotics combination therapy than patients with MBSIs（P<0.05）. Conclusion Solid malignant tumor and biliary tract infections are independent risk factors for PBSIs. These provide a basis for clinicians to identify patients at risk of PBSIs.

Key words: polymicrobial bloodstream infections, pathogen, risk factor, clinical characteristic