How should I manage community‑acquired cellulitis in a healthy adult, including antibiotic selection for non‑purulent versus purulent infection, dosing, duration, and criteria for hospital admission?

Management of Community-Acquired Cellulitis in Healthy Adults

First-Line Antibiotic Selection for Non-Purulent Cellulitis

Beta-lactam monotherapy is the standard of care for typical uncomplicated cellulitis, achieving 96% clinical success because MRSA is an uncommon cause even in high-prevalence settings. 1, 2

Recommended Oral Agents (Choose One)

• Cephalexin 500 mg orally every 6 hours provides excellent coverage against beta-hemolytic streptococci (especially Streptococcus pyogenes) and methicillin-sensitive Staphylococcus aureus, the primary pathogens in 85% of cases 1, 2
• Dicloxacillin 250–500 mg orally every 6 hours offers comparable efficacy with similar streptococcal and MSSA activity 1, 2
• Amoxicillin 500 mg orally three times daily is equally effective for typical non-purulent cellulitis 1, 2
• Penicillin V 250–500 mg orally four times daily remains appropriate when targeting streptococcal infection 1

Intravenous Options for Hospitalized Patients

• Cefazolin 1–2 g IV every 8 hours is the preferred IV beta-lactam for patients requiring hospitalization 1, 2
• Nafcillin 2 g IV every 6 hours or oxacillin 2 g IV every 6 hours are alternative IV options 1

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Treatment Duration

Treat for exactly 5 days if clinical improvement occurs (resolution of warmth/tenderness, improving erythema, afebrile); extend only if symptoms have not improved within this timeframe. 1, 2

• High-quality randomized controlled trial evidence demonstrates that 5-day courses achieve 98% clinical resolution at 14 days with no relapses by 28 days, equivalent to 10-day courses 1, 2
• Traditional 7–14-day regimens are unnecessary for uncomplicated cases and promote antimicrobial resistance without improving outcomes 1, 2

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When to Add MRSA Coverage (Purulent Cellulitis)

Add MRSA-active antibiotics ONLY when specific risk factors are present; routine MRSA coverage for typical non-purulent cellulitis represents overtreatment. 1, 2

MRSA Risk Factors Requiring Coverage

• Penetrating trauma (including injection drug use) 1, 2
• Visible purulent drainage or exudate at the infection site 1, 2
• Known MRSA colonization (nasal or prior infection) 1, 2
• Systemic inflammatory response syndrome (SIRS): fever >38°C, heart rate >90 bpm, respiratory rate >24 breaths/min 1, 2
• Failure to respond to beta-lactam therapy after 48–72 hours 1, 2

MRSA-Active Oral Regimens (When Indicated)

Option 1: Clindamycin Monotherapy

• Clindamycin 300–450 mg orally every 6 hours for 5 days provides single-agent coverage for both streptococci and MRSA, eliminating the need for combination therapy 1, 2
• Use ONLY if local MRSA clindamycin resistance is <10%; higher resistance rates make this option inappropriate 1, 2

Option 2: Combination Therapy

• Trimethoprim-sulfamethoxazole (TMP-SMX) 1–2 double-strength tablets twice daily PLUS cephalexin 500 mg every 6 hours for 5 days 1, 2
• Doxycycline 100 mg orally twice daily PLUS a beta-lactam (cephalexin or amoxicillin) for 5 days 1, 2
• Never use TMP-SMX or doxycycline as monotherapy for typical cellulitis—they lack reliable activity against beta-hemolytic streptococci, the predominant pathogens 1, 2

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Hospital Admission Criteria

Hospitalize patients when any of the following are present: 1, 2

• Systemic inflammatory response syndrome (SIRS): fever, tachycardia, hypotension, altered mental status 1
• Signs of deeper or necrotizing infection: severe pain out of proportion to examination, skin anesthesia, rapid progression over hours, "wooden-hard" subcutaneous tissue, violaceous bullae, cutaneous hemorrhage, or palpable gas 1, 2
• Severe immunocompromise or neutropenia 1
• Failure of outpatient treatment after 24–48 hours 1
• Poor adherence to therapy or inability to self-monitor 1

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Inpatient IV Antibiotic Regimens

For Uncomplicated Cellulitis Requiring Hospitalization (No MRSA Risk Factors)

• Cefazolin 1–2 g IV every 8 hours is first-line 1, 2
• Nafcillin 2 g IV every 6 hours is an alternative 1

For Severe Cellulitis with Systemic Toxicity or Suspected Necrotizing Infection

Mandatory broad-spectrum combination therapy: 1, 2

• Vancomycin 15–20 mg/kg IV every 8–12 hours (target trough 15–20 mg/L) PLUS piperacillin-tazobactam 3.375–4.5 g IV every 6 hours 1, 2
• Alternative combinations: vancomycin PLUS meropenem 1 g IV every 8 hours, OR vancomycin PLUS ceftriaxone 2 g IV daily and metronidazole 500 mg IV every 8 hours 1, 2
• Duration for complicated infections: 7–14 days, individualized based on clinical response 1, 2

For Documented Group A Streptococcal Necrotizing Fasciitis

• Penicillin PLUS clindamycin is the specific recommended combination 1

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Essential Adjunctive Measures

These interventions hasten improvement and reduce recurrence risk: 1, 2

• Elevate the affected extremity above heart level for at least 30 minutes three times daily to promote gravity drainage of edema and inflammatory substances 1, 2
• Examine interdigital toe spaces carefully for tinea pedis, fissuring, scaling, or maceration; treating these eradicates colonization and reduces recurrent infection 1, 2
• Address predisposing conditions: venous insufficiency, lymphedema, chronic edema, obesity, eczema 1, 2
• Systemic corticosteroids (prednisone 40 mg daily for 7 days) could be considered in non-diabetic adults, though evidence is limited (weak recommendation, moderate evidence) 1

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Prophylaxis for Recurrent Cellulitis

For patients with 3–4 episodes per year despite treating predisposing factors, consider prophylactic antibiotics: 1

• Oral penicillin V or erythromycin twice daily for 4–52 weeks, OR 1
• Intramuscular benzathine penicillin every 2–4 weeks 1
• Continue prophylaxis as long as predisposing factors persist 1

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Critical Pitfalls to Avoid

• Do NOT routinely add MRSA coverage for typical non-purulent cellulitis without specific risk factors—MRSA is uncommon in classic cellulitis even in high-prevalence settings, and unnecessary coverage promotes resistance 1, 2, 3
• Do NOT use doxycycline or TMP-SMX as monotherapy for typical cellulitis—they lack reliable streptococcal coverage and represent a fundamental treatment error 1, 2
• Do NOT automatically extend therapy to 7–10 days based solely on residual erythema; extend only if warmth, tenderness, or erythema have not improved after 5 days 1, 2
• Do NOT delay surgical consultation when any signs of necrotizing infection are present—prompt debridement is essential for favorable outcomes 1, 2
• Do NOT obtain blood cultures for typical uncomplicated cellulitis—they are positive in only ~5% of cases and represent unnecessary resource utilization 1, 4

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Reassessment and Treatment Failure

Reassess patients within 24–48 hours to verify clinical response; treatment failure rates of 21% have been reported with some oral regimens. 1, 2

If No Improvement After 48–72 Hours of Appropriate Therapy, Consider:

• Resistant organisms (MRSA): add empiric MRSA coverage immediately 1, 5
• Undrained abscess: perform bedside ultrasound; purulent collections require incision and drainage as primary treatment 1, 2
• Deeper infection: necrotizing fasciitis, septic arthritis, osteomyelitis 1, 2
• Alternative diagnoses: deep vein thrombosis, venous stasis dermatitis, contact dermatitis, eczema, lymphedema, erythema migrans 4, 6

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Penicillin Allergy Management

• For non-immediate hypersensitivity reactions (e.g., maculopapular rash), cephalexin remains acceptable because cross-reactivity is only 2–4%; avoid cephalexin in confirmed immediate-type amoxicillin allergy due to identical R1 side chains 2
• For true penicillin allergy, use clindamycin 300–450 mg orally every 6 hours (if local MRSA clindamycin resistance <10%) 1, 2