What is the best next step in managing a patient with a suspected skin and soft tissue infection of the lower extremity?

Best Next Step in Management for Suspected Skin and Soft Tissue Infection

The best next step in management for this patient with a suspected skin and soft tissue infection is soft tissue ultrasound (option A). 1, 2

Clinical Assessment and Diagnosis

This 55-year-old man presents with classic signs of a skin and soft tissue infection (SSTI):

• History of insect bite preceding symptoms
• Fever (38.3°C/101°F)
• Tachycardia (120/min)
• Area of induration, fluctuance, and erythema on posterior left calf
• Systemic symptoms suggesting infection

The presence of fluctuance on examination strongly suggests an abscess formation that may require drainage 3.

Rationale for Soft Tissue Ultrasound

Ultrasound is the preferred initial imaging modality in this case for several reasons:

• Rapid bedside assessment: Ultrasound can be performed quickly at the bedside to differentiate between simple cellulitis and abscess requiring drainage 2

• High diagnostic accuracy: Ultrasound has demonstrated sensitivity of 88.2%, specificity of 93.3%, and diagnostic accuracy of 91.9% in detecting soft tissue infections with fluid collections 3

• Guides management decisions: Ultrasound findings directly inform whether incision and drainage is needed, which is the definitive treatment for abscesses 1, 2

• Cost-effective and non-invasive: Compared to other imaging modalities, ultrasound is less expensive and does not expose the patient to radiation 3

Management Algorithm

1. Initial imaging with soft tissue ultrasound to:

   • Confirm presence and extent of abscess 3
   • Evaluate depth of infection 1
   • Rule out deeper tissue involvement 3

2. If abscess is confirmed:

   • Proceed with incision and drainage 3
   • Obtain cultures of the abscess material 3
   • Start empiric antibiotics covering MRSA given patient's risk factors (diabetes, systemic symptoms) 4, 5

3. If ultrasound shows concerning features for necrotizing infection:

   • Immediate surgical consultation 3
   • Broad-spectrum antibiotics 3
   • Consider advanced imaging (MRI or CT) 3

4. If ultrasound shows simple cellulitis without abscess:

   • Initiate appropriate antibiotics 3
   • Consider admission given patient's systemic symptoms 3

Why Other Options Are Less Appropriate

• MRI of left lower extremity (B): While MRI provides excellent soft tissue detail and is the imaging modality of choice for necrotizing fasciitis, it is time-consuming, expensive, and not necessary as the first-line imaging study when ultrasound can provide immediate information 3

• X-ray of left tibia and fibula (C): Has limited utility for soft tissue infections unless there is concern for osteomyelitis or gas in tissues 3

• Deep venous ultrasonography (D): Would be indicated if deep vein thrombosis was suspected, but the clinical picture strongly suggests infection rather than thrombosis 3

• CT-angiography of left lower extremity (E): Would be indicated for vascular compromise or if compartment syndrome was suspected, which is not suggested by the clinical presentation 3

Special Considerations

• The patient's diabetes is a risk factor for more severe infections and poorer outcomes, warranting close monitoring 6

• If the ultrasound reveals an abscess, incision and drainage remains the cornerstone of treatment 3

• Given the patient's systemic symptoms (fever, tachycardia), empiric antibiotic therapy should include coverage for MRSA (e.g., vancomycin) 4, 5

• If necrotizing fasciitis is suspected based on ultrasound findings or clinical deterioration, immediate surgical consultation and broader antibiotic coverage are essential 3