Predictors of Monomicrobial Necrotizing Soft Tissue Infections.

Background: Broad-spectrum antibiotic therapy is critical in the management of necrotizing soft tissue infections (NSTI) in the emergency setting. Clindamycin often is included empirically to cover monomicrobial gram-positive pathogens but probably is of little value for polymicrobial infections and is associated with significant side effects, including the induction of Clostridium difficile colitis. However, there have been no studies predicting monomicrobial infections prior to obtaining cultures. The purpose of this study was to identify independent predictors of monomicrobial NSTI where the use of clindamycin would be most beneficial. We hypothesized that monomicrobial infections are characterized by involvement of the upper extremities and fewer co-morbid diseases.

Methods: We reviewed all cases of potential NSTI occurring between 1996 and 2013 in a single tertiary-care center. The infection was diagnosed by the finding of rapidly progressing necrotic fascia during debridement with positive cultures of tissue. Univariable analysis was performed using the Student t-, Wilcoxon rank sum, χ2, and Fisher exact tests as appropriate. Multivariable logistic regression was used to identify independent variables associated with outcomes.

Results: A group of 151 patients with confirmed NSTI with complete data was used. Of the monomicrobial infections, 61.8% were caused by Group A streptococci, 20.1% by Staphylococcus aureus, and 12.7% by Escherichia coli. Of the polymicrobial infections, E. coli was involved 13.7% of the time, followed by Candida spp. at 12.9%, and Bacteroides fragilis at 11.3%. On univariable analysis, immunosuppression, upper extremity infection, and elevated serum sodium concentration were associated with monomicrobial infection, whereas morbid obesity and a perineal infection site were associated with polymicrobial infection. On multivariable analysis, the strongest predictor of monomicrobial infection was immunosuppression (odds ratio [OR] 7.0; 95% confidence interval [CI] 2.2-22.3) followed by initial serum sodium concentration (OR 1.1; 95% CI 1.0-1.2). Morbid obesity (OR 0.1; 95% CI 0.0-0.5) and perineal infection (OR 0.3; 95% CI 0.1-0.8) were independently associated with polymicrobial infection.

Conclusion: We identified independent risk factors that may be helpful in differentiating monomicrobial from polymicrobial NSTI. We suggest empiric clindamycin coverage be limited to patients who are immunosuppressed, have an elevated serum sodium concentration, or have upper extremity involvement and be avoided in obese patients or those with perineal disease.