Eschar with cellulitis as a clinical predictor in community-acquired methicillin-resistant Staphylococcus aureus (MRSA) skin abscess

This study was designed to determine the validity of a central eschar with surrounding cellulitis as a clinical predictor for CA-MRSA infection. In this 10-month prospective observational study, patients with a chief complaint or clinical findings of skin infection with abscess had study data sheets placed on their chart. All abscesses were treated with incision and drainage, and wound cultures were obtained. Exclusionary criteria included patient age under 18 years, recently incarcerated within 14 days, and hospitalized or in a nursing home within 10 days. Correlation of wound culture results with recorded physical examination determined the sensitivity, specificity, and positive/negative predictive values. A total of 224 patients with abscesses were enrolled; 18 patients met exclusion criteria. An additional 78 patients were excluded because no wound cultures had been obtained, study data form was incomplete, or there was no evidence of wound cellulitis. Of the 128 remaining patients, 91 wound cultures grew MRSA (71% prevalence). Of these 91 cases, 50 tested positive for central black eschar, yielding a sensitivity of 55% (95% confidence interval [CI] 0.45-0.65). Thirty-seven patients had abscesses that grew non-MRSA bacteria. Three of these were positive for central black eschar, yielding a specificity of 92% (95% CI 0.83-1.01). The positive predictive value was 94% (95% CI 0.88-1.01) and the negative predictive value was 45% (95% CI 0.32-0.59). A central black eschar with cellulitis has good specificity and high positive predictive value in diagnosing CA-MRSA infection.