What are the appropriate diagnostic tests and treatment options for a suspected foot infection?

Diagnostic Workup for Suspected Foot Infection

For any suspected foot infection, immediately obtain plain radiographs, perform a probe-to-bone test if an open wound is present, and send tissue cultures (not swabs) before starting antibiotics. 1

Initial Clinical Assessment

Classify infection severity at presentation by assessing the following 1:

• Vital signs and systemic toxicity: fever, hypotension, confusion, tachycardia 1
• Wound depth: probe the debrided wound to assess involvement of subcutaneous tissue, fascia, tendon, muscle, joint, or bone 1
• Extent of cellulitis: measure erythema extending >2 cm from ulcer margin 1
• Local signs: crepitus, bullae, necrosis, gangrene, purulent drainage 1

Laboratory Tests to Order

Obtain the following blood tests 1:

• C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) or procalcitonin (PCT) - particularly useful when clinical examination is equivocal 1
• Complete blood count with differential 1
• Basic metabolic panel (assess for hyperglycemia, acidosis, azotemia) 1
• Blood cultures if systemic signs present 1

Do not use foot temperature measurements or quantitative microbial analysis for diagnosis 1

Microbiological Sampling

Obtain cultures before starting antibiotics 1, 2:

• Tissue specimens are strongly preferred over swabs - obtain by curettage, biopsy, or aspiration after debridement 1, 2
• Use conventional (not molecular) microbiology techniques for first-line pathogen identification 1
• Send specimens promptly in sterile containers with clinical information 1

Imaging Studies

Plain radiographs are mandatory for all non-superficial foot infections to assess for 1:

• Bone destruction or deformity
• Soft tissue gas
• Radio-opaque foreign bodies

When to Obtain Advanced Imaging

Order MRI when 1:

• Osteomyelitis diagnosis remains uncertain despite clinical findings, plain X-rays, and inflammatory markers 1
• Deep soft tissue abscess is suspected 1

Alternative imaging if MRI unavailable or contraindicated 1:

• Leukocyte scintigraphy combined with bone scan 1
• PET scan or SPECT/CT 1

Diagnosing Osteomyelitis

Use a combination approach 1:

• Probe-to-bone test: negative test in low-risk patients largely rules out osteomyelitis; positive test in high-risk patients is largely diagnostic 1
• Plain X-rays plus ESR or CRP or PCT as initial studies 1
• Bone biopsy (percutaneous or intraoperative) for culture and histology provides definitive diagnosis and guides antibiotic selection 1, 3

Avoid using soft tissue or sinus tract cultures for osteomyelitis as they do not accurately reflect bone pathogens 1

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Treatment Approach

Hospitalization Criteria

Consider hospitalization for 1:

• Severe infection by IWGDF/IDSA classification 1
• Moderate infection with comorbidities: metabolic instability, critical ischemia, extensive cellulitis, deep abscess 1

Surgical Consultation

Obtain urgent surgical consultation within 24-48 hours for 1, 3:

• Severe infections or moderate infections with extensive gangrene 1
• Deep (subfascial) abscess 1
• Necrotizing soft tissue infection 1
• Compartment syndrome 1
• Severe lower limb ischemia requiring revascularization 1

Consider early surgery (within 24-48 hours) combined with antibiotics for moderate and severe infections to remove infected and necrotic tissue 1

Antibiotic Therapy

Empiric Regimen Selection

For mild to moderate infections (previously untreated) 1, 2:

• Target aerobic gram-positive cocci (Staphylococcus aureus, streptococci) 1, 2
• Oral options: dicloxacillin, cephalexin, or clindamycin 4

For severe infections or those with risk factors 1, 2:

• Broad-spectrum coverage initially 1
• Piperacillin-tazobactam 3.375 grams IV every 6 hours is FDA-approved for diabetic foot infections 5
• Alternative: ciprofloxacin plus clindamycin, or imipenem/cilastatin 4

Consider MRSA coverage if 1:

• Prior MRSA history
• High local MRSA prevalence
• Clinically severe infection

Do not empirically cover Pseudomonas aeruginosa in temperate climates unless previously isolated or patient resides in Asia/North Africa with moderate-severe infection 1

Duration of Antibiotic Therapy

Soft tissue infections 1:

• 1-2 weeks for mild to moderate infections 1
• Up to 3-4 weeks if extensive, slow resolution, or severe peripheral artery disease present 1
• Re-evaluate after 4 weeks if no improvement despite appropriate therapy 1

Osteomyelitis 1:

• Up to 3 weeks after minor debridement 1
• 4-6 weeks if bone not completely resected 2, 4
• Shorter duration acceptable if all infected bone removed 2

Route of Administration

Parenteral therapy initially for 1:

• All severe infections 1
• Some moderate infections 1

Switch to oral when 1:

• Patient systemically well
• Culture results available
• Highly bioavailable oral agents can be used for most mild and many moderate infections, including some osteomyelitis cases 2

Wound Care

Essential components 1, 2:

• Sharp debridement of callus, necrotic tissue, and debris 1
• Pressure off-loading (critical for healing) 1, 3
• Assess and optimize arterial perfusion 1

Do not treat clinically uninfected ulcers with antibiotics 1

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Common Pitfalls to Avoid

• Delayed multidisciplinary referral increases amputation risk 3
• Relying on soft tissue cultures for osteomyelitis leads to inappropriate antibiotic selection 3
• Failing to assess vascular status results in poor outcomes as adequate perfusion is essential for healing and antibiotic delivery 3
• Treating without proper off-loading will likely result in treatment failure 3
• Continuing antibiotics until complete wound healing is unnecessary - stop when infection resolves 1