Background/Purpose:  Little is known about patients with polymicrobial septic arthritis (SA) and how they may differ from those with SA due to a single organism. The purpose of this study is to define the demographic features, clinical characteristics, microbiology, and outcomes of patients with polymicrobial SA as compared to those with monomicrobial SA.

Methods:  We conducted a retrospective cohort study of patients 18 years and older admitted to a single tertiary care center from 1998 to 2015 with culture-positive, surgically treated SA affecting one or more joints. Baseline characteristics, clinical features, microbial profile, rate of operative intervention, length of hospital stay (LOS), and 60-day readmission rates were determined. Patients were stratified by presence of SA due to a single organism or more than one organism, identified either in the blood of patients with inflammatory arthritis or in synovial fluid.

Results:  Of 511 patients with SA, 52 (10.2%) were found to have polymicrobial SA. Demographic features were similar in the polymicrobial and monomicrobial SA groups. Of those with polymicrobial SA, 6 had polyarticular involvement, similar to the rate of polyarticular involvement in those with monomicrobial SA. Polymicrobial SA was less commonly preceded by joint trauma (5.9% versus 18.1%; p = 0.03). Those with polymicrobial SA tended to have lower synovial fluid white blood cell counts (SF WBC) (71.2 versus 101.4 x 10⁹ per liter) with significantly lower % polymorphonuclear (PMN) cells (78.1% versus 88.0%; p = 0.003). Polymicrobial SA occurred most frequently in the knee (n = 27) and hip (n = 13), but no differences in joint distribution were found as compared to those with SA due to a single organism. Coagulase negative Staphylococcus (32.7% versus 12.0%; p < 0.001), Enterococci (33% versus 2.5%, p <0.001) and Escherichia Coli ( 15% versus 2.5%, p <0.001) are the three most frequently isolated organism in polymicrobial SA (Figure 1). LOS was longer in those with polymicrobial SA (13.4 versus 10.7 days; p = 0.04), and there was a trend toward more frequent Intensive Care Unit (ICU) stays (25.0% versus 15.9%; p = 0.098). Rates of endocarditis were similar in patients with polymicrobial and monomicrobial SA.

Conclusion:  Patients with polymicrobial SA may differ in important ways from patients with monomicrobial SA. Despite lower SF WBC counts and many infections with bacteria of low virulence, patients with polymicrobial SA may have more severe disease, as suggested by a longer LOS and more frequent transfers to the ICU.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/distinguishing-features-of-polymicrobial-septic-arthritis/