What is the recommended outpatient antibiotic regimen, duration, and criteria for hospitalization for foot cellulitis in an otherwise healthy adult without recent MRSA infection or known colonization?

Foot Cellulitis: Outpatient Management

First-Line Antibiotic Regimen

For an otherwise healthy adult with foot cellulitis and no recent MRSA infection or colonization, prescribe a beta-lactam antibiotic such as cephalexin 500 mg orally every 6 hours for 5 days. 1

• Beta-lactam monotherapy is the standard of care for typical uncomplicated cellulitis, with a 96% success rate. 1
• MRSA is an uncommon cause of typical nonpurulent cellulitis, even in high-prevalence settings, making routine MRSA coverage unnecessary. 1, 2
• Recommended oral beta-lactam options include cephalexin, dicloxacillin (250-500 mg every 6 hours), amoxicillin, or penicillin V (250-500 mg four times daily). 1

Treatment Duration

Treat for exactly 5 days if clinical improvement occurs—defined as resolution of warmth and tenderness with improving erythema. 1

• Extend treatment beyond 5 days only if symptoms have not improved within this timeframe. 1
• Traditional 7-14 day courses are no longer necessary for uncomplicated cases. 1
• Reassess within 24-48 hours to verify clinical response, as treatment failure rates of 21% have been reported with some oral regimens. 1

When MRSA Coverage Is NOT Needed

Do not add MRSA-active antibiotics for typical nonpurulent foot cellulitis in your patient population. 1

• The combination of trimethoprim-sulfamethoxazole plus cephalexin is no more efficacious than cephalexin alone in pure cellulitis without abscess, ulcer, or purulent drainage. 1, 3
• Adding MRSA coverage to beta-lactam therapy provides no additional benefit in typical cases. 1

When to Add MRSA Coverage

Add MRSA-active antibiotics only when specific risk factors are present: 1

• Penetrating trauma or injection drug use 1
• Purulent drainage or exudate (visible pus) 1
• Known MRSA colonization or prior MRSA infection 1
• Systemic inflammatory response syndrome (SIRS): fever >38°C, tachycardia >90 bpm, tachypnea >24 rpm 1
• Failure to respond to beta-lactam therapy after 48-72 hours 1

If MRSA coverage is needed, use clindamycin 300-450 mg orally every 6 hours as monotherapy (covers both streptococci and MRSA), but only if local clindamycin resistance rates are <10%. 1

• Alternative regimens include trimethoprim-sulfamethoxazole (1-2 double-strength tablets twice daily) plus a beta-lactam, or doxycycline (100 mg twice daily) plus a beta-lactam. 1
• Never use doxycycline or trimethoprim-sulfamethoxazole as monotherapy for typical cellulitis—they lack reliable activity against beta-hemolytic streptococci, the primary pathogens. 1

Criteria for Hospitalization

Hospitalize if any of the following are present: 1

• Systemic inflammatory response syndrome (SIRS): fever, tachycardia, tachypnea 1
• Hypotension or hemodynamic instability 1
• Altered mental status or confusion 1
• Severe immunocompromise or neutropenia 1
• Signs of necrotizing infection: severe pain out of proportion to examination, skin anesthesia, rapid progression, "wooden-hard" subcutaneous tissues, gas in tissue, or bullous changes 1

Essential Adjunctive Measures

Elevate the affected foot above heart level for at least 30 minutes three times daily to promote gravity drainage of edema. 1

• Examine interdigital toe spaces for tinea pedis, fissuring, scaling, or maceration—treating these eradicates colonization and reduces recurrent infection. 1
• Address predisposing conditions including venous insufficiency, lymphedema, and chronic edema. 1

Common Pitfalls to Avoid

• Do not reflexively add MRSA coverage simply because the infection is on the foot—typical foot cellulitis without risk factors responds to beta-lactams in 96% of cases. 1
• Do not extend treatment to 7-10 days based on residual erythema alone—some inflammation persists even after bacterial eradication. 1
• Do not use doxycycline or trimethoprim-sulfamethoxazole as monotherapy—these agents miss streptococcal pathogens that cause the vast majority of cellulitis cases. 1
• Assess for abscess with ultrasound if there is any clinical uncertainty—purulent collections require incision and drainage plus MRSA-active antibiotics, not beta-lactams alone. 1

Treatment Failure Management

If the patient fails to improve after 48-72 hours on appropriate beta-lactam therapy: 1

• Reassess for MRSA risk factors, necrotizing infection, or misdiagnosis 1
• Switch to vancomycin 15-20 mg/kg IV every 8-12 hours or linezolid 600 mg IV twice daily if MRSA is suspected or the patient has systemic toxicity 1
• Obtain emergent surgical consultation if necrotizing fasciitis is suspected 1