Infectious Disease · Topic 6 of 10
Cellulitis & Skin Abscess
Infectious disease · skin and soft tissue
Cellulitis & Skin Abscess
Purulent or not? Drain every abscess and cover MRSA when systemic; treat nonpurulent cellulitis for streptococci — usually for 5 days.
Key numbers
Treat cellulitis for 5 days5 days
Systemic signs set severity: mild, moderate, severeT > 38 °C · HR > 90
01 · DoIncision and drainage for every abscess
Incision and drainage is the primary treatment for abscesses, carbuncles and large furuncles; send pus for Gram stain and culture (mainly to find MRSA), and drain painful swelling without waiting for fluctuance.
02 · Don't confusePurulent = S. aureus; nonpurulent = streptococci
Classify every SSTI as purulent (abscess, furuncle, carbuncle, purulent drainage) — usually S. aureus, often MRSA — or nonpurulent (cellulitis, erysipelas) — usually β-hemolytic streptococci; the category drives treatment.
03 · The ruleTreat cellulitis for 5 days
The recommended antibiotic duration for uncomplicated cellulitis is 5 days; extend it only if the infection has not improved within that time (IDSA, strong recommendation).
01
Causes & risk factors
Why it happensNonpurulent with MRSA risk: add MRSA coverageCover MRSA plus streptococci (eg, vancomycin) in nonpurulent cellulitis with penetrating trauma, injection drug use, MRSA infection elsewhere, nasal MRSA colonization or SIRS.
02
Presentation
What you'll seePain out of proportion: think necrotizing infectionPain out of proportion, bullae, skin sloughing or anesthesia, cutaneous hemorrhage, tissue gas, rapid spread or hypotension suggest a necrotizing infection — get emergent surgical evaluation (see Necrotizing Soft-Tissue Infection).
03
Diagnosis & monitoring
Work-upSystemic signs set severity: mild, moderate, severeIDSA SSTI severity: mild = no systemic signs; moderate = SIRS signs (T > 38 °C or < 36 °C, HR > 90, RR > 24, WBC > 12,000 or < 4,000/µL); severe = failed oral therapy, immunocompromise or deeper-infection signs (bullae, hypotension, organ dysfunction).
Blood cultures only for high-risk cellulitisBlood, aspirate and tissue cultures are not routine in cellulitis; obtain them for patients on chemotherapy, with neutropenia or severe cell-mediated immunodeficiency, and after immersion injuries or animal bites.
04
Do right now
In this orderFirst moves
1
→Systemic signs set severity: mild, moderate, severeT > 38 °C · HR > 90
2
→Pain out of proportion: think necrotizing infection
3
→Incision and drainage for every abscess
4
Purulent with systemic signs: cover MRSA
Why each step
Systemic signs set severity: mild, moderate, severeIDSA SSTI severity: mild = no systemic signs; moderate = SIRS signs (T > 38 °C or < 36 °C, HR > 90, RR > 24, WBC > 12,000 or < 4,000/µL); severe = failed oral therapy, immunocompromise or deeper-infection signs (bullae, hypotension, organ dysfunction).
Pain out of proportion: think necrotizing infectionPain out of proportion, bullae, skin sloughing or anesthesia, cutaneous hemorrhage, tissue gas, rapid spread or hypotension suggest a necrotizing infection — get emergent surgical evaluation (see Necrotizing Soft-Tissue Infection).
Incision and drainage for every abscessIncision and drainage is the primary treatment for abscesses, carbuncles and large furuncles; send pus for Gram stain and culture (mainly to find MRSA), and drain painful swelling without waiting for fluctuance.
Purulent with systemic signs: cover MRSAPurulent infection with systemic signs: oral TMP-SMX DS twice daily or doxycycline 100 mg twice daily (moderate); IV vancomycin — or daptomycin, linezolid or ceftaroline — if severe or drainage plus oral therapy failed.
05
Management
OrdersMild cellulitis: oral cephalexin covers strep must knowMild nonpurulent cellulitis without systemic signs: an oral agent active against streptococci — cephalexin, dicloxacillin or penicillin VK; clindamycin for penicillin allergy. Treat as an outpatient.
Systemic signs, nonpurulent: IV cefazolin or ceftriaxoneNonpurulent cellulitis with systemic signs (moderate): IV cefazolin, ceftriaxone, penicillin or clindamycin; many clinicians include coverage of methicillin-susceptible S. aureus.
Severe or deep: vancomycin plus piperacillin-tazobactamSevere nonpurulent infection in a severely compromised patient or with signs of deeper infection: vancomycin plus piperacillin-tazobactam or imipenem/meropenem, with prompt surgical consultation to exclude necrotizing infection.
Antibiotics after drainage for complex or systemic abscessesAfter drainage, add an anti-staphylococcal antibiotic for systemic signs, surrounding cellulitis, multiple abscesses, immunocompromise or a facial abscess; where MRSA is prevalent, adding one for any drained abscess lowers failure and recurrence.
Elevate the limb and treat toe-web tineaElevate the affected area and treat predisposing conditions: examine the toe webs for fissuring, scaling or maceration (tinea), and manage edema, venous insufficiency, obesity and eczema to prevent recurrence.
06
The rules
Know theseAdmit for SIRS, instability or deep infectionHospitalize for SIRS, altered mental status or hemodynamic instability, suspected deep or necrotizing infection, severe immunocompromise, poor adherence or failed outpatient therapy; otherwise treat as an outpatient.
07
Purulent or nonpurulent SSTI?
Commonly confused| Examples | Abscess, furuncle, carbuncle |
|---|---|
| Usual organism | S. aureus, often MRSA |
| Core treatment | Incision and drainage |
| Mild, oral | Drainage alone; add TMP-SMX or doxycycline if systemic signs |
| Severe | IV vancomycin plus drainage |
08
Commonly confused
Tell apartBilateral red legs: think stasis dermatitisCellulitis is usually unilateral, warm and erythematous with peau d'orange and lymphangitis; bilateral involvement suggests stasis dermatitis, and DVT and contact dermatitis are the other key mimics.
09
More detail
ReferenceShow 6 reference facts
Point-of-care ultrasound helps distinguish an abscess from cellulitis when fluctuance is uncertain; routine irrigation or packing is unnecessary for abscesses smaller than 5 cm.
Cellulitis with neutropenia needs broad coverage — vancomycin plus cefepime or meropenem — and tissue culture or biopsy, because gram-negative and fungal pathogens are more likely.
Recurrent abscesses: look for a local cause (pilonidal cyst, hidradenitis suppurativa, foreign body), drain and culture early, treat 5–10 days, and consider a 5-day decolonization with intranasal mupirocin plus chlorhexidine washes.
With 3–4 cellulitis episodes a year despite managing risk factors, consider prophylactic oral penicillin or erythromycin twice daily (4–52 weeks) or IM benzathine penicillin every 2–4 weeks.
Animal-bite cellulitis is polymicrobial (Pasteurella from cats and dogs) — treat with amoxicillin-clavulanate; after salt or brackish water exposure (Vibrio), add doxycycline to a cephalosporin plus fluoroquinolone.
IDSA 2014 makes a weak suggestion that systemic corticosteroids (eg, prednisone 40 mg daily for 7 days) could be considered in nondiabetic adults with cellulitis.
Pus means staph until proven otherwise — and a collection needs a blade, not just a pill.
Sources · Stevens DL et al. Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the IDSA. Clin Infect Dis 2014;59(2):e10–e52 (open copy read: spice.unc.edu) · Merck Manual Professional — Cellulitis (May 2026) · Merck Manual Professional — Cutaneous Abscess (May 2026) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).