Low Frequency of Staphylococcus Aureus in Lower Extremity Skin and Soft Tissue Infections

Background: Staphylococcus aureus is often implicated in skin/soft tissue infections (SSTI). However, SSTI at sites of pressure necrosis and peripheral vascular disease (PVD) are often polymicrobial. The frequency of S aureus in these infections is uncertain. Methods: We retrospectively reviewed culture results from adults (January 1, 2015-March 31, 2017), evaluated their records and selected SSTI in lower extremities. The patient demographics, comorbidities, characteristics and culture results were recorded. The results were stratified by S aureus status and a composite risk score (RS) was developed (2 points for each difference in S aureus frequency with P < 0.05 [chi-square test] and 1 point for P = 0.06-0.1). The predictors of S aureus were determined by regression analysis using SSPS software. Results: We encountered 356 lower extremity-SSTI (243 foot/ankle, 56 tibia/calf, 30 thigh, 12 hip and 15 groin). S aureus was detected in 173 (48.6%) cases, 59.6% were methicillin-resistant isolates. S aureus was more common in lesions without necrosis (56.3% vs. 42.9%; P = 0.01), with drainage (59.6% vs. 44.7%; P = 0.02), in male sex (53.2% vs. 40.0%; P = 0.02) and was less common in patients with PVD (38.1% vs. 50.9%; P = 0.07), and paraplegia (39.6% vs. 50.0%; P = 0.2). S aureus was less common in polymicrobial SSTI (45.0% vs. 58.5%; P = 0.03). RS of 0-8 correlated with increasing S aureus prevalence from 23.1% (RS = 0-1) to 78.6% (RS = 8; P < 0.001). The predictors of S aureus were drainage (odds ratio [OR] = 1.83; 95% confidence intervals [CI]: 1.11, 3.02), lack of PVD (OR = 1.59; CI: 1.03, 2.46) and absence of necrosis (OR = 1.91; CI: 1.08, 3.40). Conclusions: Patients with suspected polymicrobial lower extremity-SSTI and low RS may not need empirical antistaphylococcal therapy.