How should purulent cellulitis be treated, including appropriate antibiotic selection for outpatient and inpatient care and the need for incision and drainage?

Treatment of Purulent Cellulitis

For purulent cellulitis, incision and drainage is the primary treatment when a drainable collection is present, and empiric MRSA-active antibiotics should be added for patients with systemic signs, multiple lesions, immunocompromise, or surrounding cellulitis. 1

Distinguishing Purulent from Non-Purulent Cellulitis

The critical first step is determining whether the infection is purulent (with visible drainage, exudate, or abscess) or non-purulent (typical cellulitis without purulence). 1

• Purulent cellulitis requires MRSA coverage because community-acquired MRSA is the predominant pathogen in these infections 2, 3, 4
• Non-purulent cellulitis is typically caused by beta-hemolytic streptococci and does NOT require routine MRSA coverage—beta-lactam monotherapy achieves 96% success 5
• Use point-of-care ultrasound when clinical examination is equivocal to identify drainable collections 2

Incision and Drainage: The Cornerstone of Treatment

For any purulent collection (abscess, furuncle, carbuncle), incision and drainage is the definitive treatment. 1

• I&D alone may be sufficient for simple abscesses without surrounding cellulitis in immunocompetent patients 1, 2
• Loop drainage technique is preferred over traditional I&D for better outcomes 2
• Add systemic antibiotics after drainage when:
  • Systemic inflammatory response is present (fever >38°C, tachycardia >90 bpm, tachypnea >24 breaths/min) 1
  • Multiple infection sites exist 1
  • Extensive surrounding cellulitis (erythema >5 cm from abscess) 1
  • Immunocompromise, diabetes, or extremes of age 1
  • Difficult-to-drain locations (face, hands, genitalia) 1
  • Lack of response to drainage alone 1

Outpatient Oral Antibiotic Regimens for Purulent Cellulitis

When antibiotics are indicated, choose ONE of the following MRSA-active regimens for 5 days (extend only if no improvement): 5

First-Line Options:

• Trimethoprim-sulfamethoxazole (TMP-SMX) 1-2 double-strength tablets twice daily 1, 5

  • Must be combined with a beta-lactam (cephalexin or amoxicillin) if any non-purulent cellulitis component exists, as TMP-SMX lacks reliable streptococcal coverage 5
  • Avoid in third-trimester pregnancy and infants <2 months 5

• Doxycycline 100 mg twice daily 1, 5

  • Also requires combination with a beta-lactam for mixed purulent/non-purulent presentations 5
  • Absolutely contraindicated in children <8 years (permanent tooth discoloration, impaired bone growth) 5
  • Pregnancy category D—avoid in pregnant women 5

• Clindamycin 300-450 mg every 6-8 hours 1, 5

  • Provides single-agent coverage for both MRSA and streptococci 5
  • Use ONLY if local MRSA clindamycin resistance is <10% 1, 5
  • Higher risk of Clostridioides difficile infection 5

• Linezolid 600 mg twice daily 5

  • Effective but expensive; typically reserved for complicated cases 5

Inpatient IV Antibiotic Regimens

Hospitalize patients with purulent cellulitis who have: 5

• Systemic inflammatory response syndrome (SIRS) 1, 5
• Hypotension or hemodynamic instability 5
• Altered mental status 5
• Severe immunocompromise or neutropenia 5
• Failed outpatient therapy 5

IV MRSA-Active Agents (all have A-I level evidence): 5

• Vancomycin 15-20 mg/kg IV every 8-12 hours (target trough 15-20 mg/L) 1, 5
• Linezolid 600 mg IV twice daily 5
• Daptomycin 4 mg/kg IV once daily 5
• Clindamycin 600 mg IV every 8 hours (only if local resistance <10%) 5

Treatment duration for complicated infections: 7-14 days, individualized by clinical response 5

Severe Infections Requiring Broad-Spectrum Coverage

For patients with signs of systemic toxicity, rapid progression, or suspected necrotizing fasciitis, mandatory broad-spectrum combination therapy includes: 5

• Vancomycin 15-20 mg/kg IV every 8-12 hours PLUS 5
• Piperacillin-tazobactam 3.375-4.5 g IV every 6 hours 5

Alternative combinations: vancomycin + carbapenem OR vancomycin + ceftriaxone + metronidazole 5

Obtain emergent surgical consultation immediately if necrotizing infection is suspected (severe pain out of proportion, skin anesthesia, rapid progression, gas in tissue, bullae, "wooden-hard" tissue) 5, 4

Pediatric Dosing for Purulent Cellulitis

Oral Regimens:

• Clindamycin 10-13 mg/kg/dose every 6-8 hours (max 40 mg/kg/day), only if local resistance <10% 5
• TMP-SMX 4-6 mg/kg/dose (TMP component) twice daily, must add beta-lactam for streptococcal coverage 5
• Doxycycline 2 mg/kg/dose twice daily (max 100 mg/dose) for children ≥8 years and <45 kg, contraindicated <8 years 5

IV Regimens:

• Vancomycin 15 mg/kg IV every 6 hours (first-line, A-II evidence) 5
• Clindamycin 10-13 mg/kg IV every 6-8 hours if stable, no bacteremia, and local resistance <10% 5
• Linezolid 10 mg/kg IV every 8 hours (<12 years) or 600 mg IV twice daily (≥12 years) 5

Critical Pitfalls to Avoid

• Never use beta-lactams alone (cephalexin, dicloxacillin, amoxicillin) for purulent cellulitis—they lack MRSA activity 5
• Never use doxycycline or TMP-SMX as monotherapy for typical cellulitis—they don't reliably cover streptococci 5
• Never prescribe antibiotics alone for drainable abscesses—I&D is the primary treatment 1, 2
• Do not delay surgical consultation when necrotizing infection signs are present—these progress rapidly and require debridement 5, 4
• Reassess within 24-48 hours—treatment failure rates of 21% have been reported with oral regimens 5

Adjunctive Measures

• Elevate the affected extremity above heart level for at least 30 minutes three times daily to promote drainage 5
• Treat predisposing conditions: tinea pedis, venous insufficiency, lymphedema, chronic edema, obesity, eczema 5
• Examine interdigital toe spaces for fungal infection and treat to reduce recurrence 5