What are the recommended antibiotics (abx) for treating an abscess and cellulitis?

Antibiotic Treatment for Abscess and Cellulitis

Immediate Treatment Decision Algorithm

For abscess with cellulitis: Incision and drainage is the primary treatment, with antibiotics targeting MRSA added only if there is surrounding cellulitis, systemic signs, immunocompromise, or incomplete source control. 1, 2

For cellulitis without abscess: Start with beta-lactam monotherapy (cephalexin, dicloxacillin, or amoxicillin) targeting streptococci, and add MRSA coverage only if purulent drainage is present, penetrating trauma occurred, or the patient fails to improve within 48-72 hours. 1, 3

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Outpatient Oral Antibiotic Regimens

For Pure Cellulitis (Non-Purulent, No Abscess)

• First-line therapy: Cephalexin 500 mg four times daily, dicloxacillin 500 mg four times daily, or amoxicillin 500 mg three times daily for 5 days if clinical improvement occurs 1, 3
• Beta-lactam monotherapy successfully treats 96% of typical cellulitis cases, confirming that MRSA is an uncommon cause 1
• Penicillin or amoxicillin are acceptable alternatives targeting streptococci 1

For Purulent Cellulitis or Abscess with Surrounding Cellulitis

• First-line MRSA coverage: TMP-SMX 1-2 double-strength tablets twice daily OR doxycycline 100 mg twice daily for 5-10 days 2, 3, 4
• Alternative if local MRSA resistance <10%: Clindamycin 300-450 mg three times daily 1, 2, 3
• Clindamycin or TMP-SMX plus incision and drainage achieves 83% cure rates versus 69% with drainage alone for abscesses 4
• Clindamycin reduces new infection rates at 1 month (6.8%) compared to TMP-SMX (13.5%) or placebo (12.4%) 4

For Dual Coverage (Streptococci + MRSA)

• Combination therapy: TMP-SMX or doxycycline PLUS a beta-lactam (cephalexin, amoxicillin, or penicillin) 1, 3
• Monotherapy option: Clindamycin alone if local MRSA resistance is <10% 1, 2, 3
• A randomized trial demonstrated that cephalexin plus TMP-SMX was no more effective than cephalexin alone for pure cellulitis (85% vs 82% cure rates), confirming MRSA coverage is unnecessary without purulence 1, 5

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Inpatient IV Antibiotic Regimens

Indications for Hospitalization

• Systemic toxicity: fever, hypotension, tachycardia, altered mental status, or SIRS criteria 2
• Rapidly progressive infection, multiple infection sites, extremes of age, or significant comorbidities 2, 3
• Failed outpatient therapy or abscess in difficult-to-drain locations 2, 3

IV Antibiotic Selection

• Gold standard for MRSA coverage: Vancomycin 15-20 mg/kg IV every 8-12 hours 2, 3
• Equally effective alternatives: Linezolid 600 mg IV every 12 hours (A-I evidence) or daptomycin 4 mg/kg IV once daily 2, 6
• For dual coverage (MRSA + streptococci): Vancomycin plus piperacillin-tazobactam for severe infections with systemic signs 3
• Clindamycin 600 mg IV every 8 hours is acceptable only if local MRSA resistance is <10% 1, 2

Treatment Duration

• Standard duration: 5 days if clinical improvement occurs, with extension only if symptoms persist 1, 2
• Complicated infections requiring debridement: 7-14 days guided by clinical response 2, 3
• Blood cultures should be obtained in hospitalized patients with malignancy, severe systemic features, neutropenia, or severe immunodeficiency 1, 2

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Critical Risk Factors Mandating MRSA Coverage

• Purulent drainage or exudate from the infection site 2, 3
• Penetrating trauma, especially injection drug use 1, 3
• Known MRSA colonization or prior MRSA infection 3
• Failure to respond to beta-lactam therapy within 48-72 hours 1, 3
• Systemic inflammatory response syndrome (SIRS) or sepsis 3

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Transition to Oral Therapy

• Once clinical improvement is demonstrated (typically after 2-3 days of IV therapy), transition to oral antibiotics 2
• Oral options: TMP-SMX 1-2 double-strength tablets twice daily, doxycycline 100 mg twice daily, or clindamycin 300-450 mg three times daily if resistance <10% 2, 3

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Abscess Management Specifics

Simple Abscess

• Primary treatment: Incision and drainage alone without antibiotics if induration and erythema are limited to the abscess borders 1
• Antibiotics are indicated only if systemic signs are present, the patient is immunocompromised, source control is incomplete, or significant surrounding cellulitis exists 1, 2

Complex Abscess (Perianal, Perirectal, Injection Drug Sites)

• Empiric broad-spectrum therapy: Coverage for gram-positive, gram-negative, and anaerobic bacteria 1
• Surgical drainage is mandatory and should be performed promptly 1

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Common Pitfalls to Avoid

• Never use beta-lactams alone when MRSA is suspected or confirmed—they have zero activity against methicillin-resistant organisms 2, 3
• Never use TMP-SMX or doxycycline alone for pure cellulitis—they have poorly defined activity against beta-hemolytic streptococci, the most common cellulitis pathogens 1, 3
• Antibiotics alone will fail for abscess regardless of choice—drainage is the primary treatment 2
• Do not use clindamycin if local MRSA resistance exceeds 10% or if inducible resistance is detected on susceptibility testing 1, 2, 3
• Do not routinely obtain cultures for typical cellulitis—they are unnecessary and do not change management 1

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Adjunctive Measures

• Elevate the affected extremity to promote gravity drainage of edema and inflammatory substances 1, 2
• Treat predisposing conditions: tinea pedis, toe web abnormalities, venous insufficiency, lymphedema, or trauma 1, 2
• Consider systemic corticosteroids (prednisone 40 mg daily for 7 days) in non-diabetic adults with cellulitis, though evidence is limited 1, 2

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Evidence Quality Considerations

The IDSA 2014 guidelines remain the cornerstone for cellulitis management, demonstrating that beta-lactam monotherapy is successful in 96% of typical cellulitis cases 1. The 2017 placebo-controlled trial by Daum et al. provided definitive evidence that antibiotics improve abscess outcomes when combined with drainage (83% cure rate vs 69% with drainage alone) 4. However, the 2013 trial by Pallin et al. demonstrated that adding TMP-SMX to cephalexin for pure cellulitis provides no benefit (85% vs 82% cure rates), confirming that MRSA coverage is unnecessary without purulence 5. These high-quality randomized trials support a conservative approach to MRSA coverage in typical cellulitis.