Abscess volume and ultrasound characteristics of community-associated methicillin-resistant Staphylococcus aureus infection

Background: Skin abscesses may vary in volume and inflammation based on organism, although this has not been evaluated using emergency ultrasonography (EUS). Objective: The objective of this study was to examine the utility of EUS in discerning skin abscess volume and inflammation by infecting organism. Methods: This was a secondary analysis of prospectively enrolled subjects 2 months to 19 years presenting for a skin abscess. Subjects with a prior drainage procedure, multiple lesions, incomplete EUS measurements, or lack of an abscess culture were excluded. Abscess cavity dimensions in the x, y, and z planes and signs of local inflammation (cobblestoning, hyperechoic, or thickened dermis) were determined. Abscess volume was calculated using the ellipsoid formula: 4/3 P I (rx) I (ry) I (rz). Results: One hundred eighty-eight subjects met the inclusion criteria. Mean age was 7.7 T 6.2 years; 39.9% were male. The gluteal region was most commonly involved (33.0%), and lesions were present for a mean 4.2 days (95% confidence interval [CI], 3.8-4.6 days). Methicillinresistant Staphylococcus aureus (MRSA) was isolated from 125 (66.5%); methicillin- sensitive S. aureus (21.8%) was most common among non- MRSA lesions. Abscess volume was smaller in MRSA (1.12 cm3) compared with non-MRSA (2.46 cm3) lesions (mean difference, j1.33 cm3; 95% CI, j2.21 to j0.47 cm3). No differences between MRSA and non- MRSA lesions were present for EUS signs of inflammation. When adjusting for age, duration of lesion, and spontaneous drainage, smaller abscess volumes were associated with MRSA infection (odds ratio, 0.83; 95% CI, 0.71-0.97). Using an optimal threshold value of 1.32 cm3, sensitivity and specificity for non-MRSA lesion were 50.8% and 81.5%, respectively. Conclusions: Methicillin-resistant S. aureus infection is statistically negatively associated with abscess volume, although of limited predictive ability. Findings using EUS suggest that MRSA does not differ from other organisms with respect to size and inflammation. Clinicians should not consider unique treatment for the presence of MRSA abscess based on these EUS findings.