Cutaneous Abscess, Furuncle, Carbuncle
Purulent skin and soft tissue infections of the hair follicle and surrounding dermis; most caused by S. aureus, increasingly MRSA.
Also known as: abscess, boil, furuncle, carbuncle, skin abscess, MRSA, folliculitis
Overview
Purulent bacterial infections of the skin and subcutaneous tissue. Folliculitis = inflammation of a hair follicle (superficial). Furuncle (boil) = deep follicular abscess. Carbuncle = coalesced cluster of adjacent furuncles with multiple sinus tracts. Abscess (more general) = localized collection of pus within a cavity.
Epidemiology
Most common SSTI presentation in US ambulatory care; ~3 million ED visits annually. Community-acquired MRSA (USA300 strain) now accounts for >50% of purulent SSTIs in many US regions.
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Risk factors
- S. aureus colonization (nares, axillae, groin)
- Skin trauma, IV drug use, athletes (wrestling, football), crowded living, daycare, prisons, military
- Diabetes, obesity, atopic dermatitis, hidradenitis suppurativa
- Immunosuppression — HIV, transplant, neutropenia
- Prior antibiotic use (selects for MRSA)
- Foreign bodies, retained sutures, prosthetic devices
Pathophysiology
S. aureus (including MRSA) enters through follicular ostia or skin breaks → bacterial proliferation → robust neutrophilic response → liquefactive necrosis → fluctuant abscess. PVL (Panton-Valentine leukocidin) toxin in CA-MRSA contributes to tissue necrosis and recurrent disease.
Clinical presentation
Symptoms
- Painful, tender, warm swelling
- Spontaneous drainage of pus
- Mild fever and constitutional symptoms (severe systemic symptoms suggest deeper or invasive infection)
- Recurrent boils (especially MRSA) frustrating patient and family
Signs / physical exam
- Folliculitis: small (1-5 mm) erythematous papules and pustules centered on hair follicles; usually no fluctuance
- Furuncle: deep tender erythematous nodule (1-2 cm) with central pustule; eventual fluctuance and spontaneous drainage
- Carbuncle: large, deep, tender plaque with multiple draining openings on neck, back, thighs; more systemic illness
- Abscess: well-circumscribed tender fluctuant nodule with surrounding erythema; pointing pustule or already drained
- Lymphangitis (red streaking) or regional lymphadenopathy if more advanced
- Systemic signs of sepsis: tachycardia, hypotension, fever — indicates need for hospitalization and IV antibiotics
Classic findings
Tender erythematous fluctuant nodule that 'points' or drains pus; carbuncle = multifocal cluster with sinus tracts.
Differential diagnosis
- Cellulitis (non-purulent) — Diffuse warmth, erythema, edema without focal fluctuance; group A strep or MSSA; no I&D needed
- Hidradenitis suppurativa — Recurrent abscesses in intertriginous areas with sinus tracts and scarring
- Pilonidal cyst/abscess — Sacrococcygeal location with hair tuft
- Epidermoid (sebaceous) cyst — Painless mobile nodule with central punctum; if infected, manage as abscess but excise wall after resolution
- Sporotrichosis / atypical mycobacteria — Chronic nodular lymphangitic spread; gardening / aquarium / hot tub exposure
- Necrotizing fasciitis — Pain disproportionate to exam, rapid progression, crepitus, systemic toxicity, hypotension; SURGICAL EMERGENCY
- Erysipelas — Sharply demarcated raised erythematous plaque, fever, group A strep
- Tularemia / anthrax / bartonella — Specific exposure history, eschar (anthrax), regional lymphadenopathy
Diagnostic workup
Diagnostic criteria
Clinical: fluctuant tender nodule; ultrasound confirms collection; culture identifies pathogen and susceptibility.
Labs
- Most uncomplicated abscesses require no labs — I&D is diagnostic and therapeutic
- Wound culture from drained pus if: severe, treatment-failure, immunocompromised, recurrent, atypical exposure
- CBC, BMP, blood cultures if systemic illness (fever, tachycardia, hypotension)
- HbA1c if recurrent disease
- HIV testing if otherwise unexplained recurrent severe infections
Imaging
- Bedside ultrasound to distinguish cellulitis from abscess and identify pockets when clinical exam ambiguous
- CT/MRI for deep abscess, suspected necrotizing fasciitis (NF), perirectal abscess, complex anatomy
Diagnostic algorithm
flowchart TD
A[Tender erythematous nodule] --> B{Fluctuant?}
B -->|No / cellulitis| C[Empiric oral antibiotic<br/>cephalexin if strep/MSSA;<br/>add MRSA if purulent risk]
B -->|Yes / abscess| D[Incision & Drainage]
D --> E{Severity / Risk Features?}
E -->|Small <2 cm, immunocompetent, no cellulitis| F[I&D alone may suffice]
E -->|>2 cm, cellulitis, systemic Sx,<br/>immunocompromised, facial, recurrent| G[I&D + oral antibiotic<br/>TMP-SMX, doxy, or clindamycin]
E -->|Sepsis, NF concern| H[Admit / IV antibiotics<br/>vancomycin ± piperacillin-tazobactam]
H --> I[Surgical consult for necrotizing infection]
G --> J{Recurrent furunculosis?}
J -->|Yes| K[Decolonize household:<br/>intranasal mupirocin + chlorhexidine]Treatment
First-line
- Incision and drainage (I&D) is the cornerstone of therapy — single most important intervention
- • Linear incision over fluctuant point with #11 blade
- • Express purulent material, break loculations with hemostat
- • Irrigate copiously with saline
- • Loose packing for large cavities (questionable benefit for <5 cm abscesses)
- • Loop drainage and minimally invasive techniques as alternatives
- Antibiotic therapy AFTER I&D (IDSA 2014 update — initially considered unnecessary for uncomplicated):
- • Recommended for: abscess >2 cm, multiple lesions, surrounding cellulitis, systemic signs, immunocompromised, very young/old, recurrent, failure of prior I&D, facial location
- • MRSA coverage: trimethoprim-sulfamethoxazole (TMP-SMX) 1-2 DS tablets BID × 7-10 days OR doxycycline 100 mg BID OR clindamycin 300-450 mg QID (check D-test for inducible resistance)
- • MSSA-only: cephalexin 500 mg QID or dicloxacillin 500 mg QID
- • Empiric for severe: IV vancomycin, linezolid, daptomycin, or ceftaroline
- Warm compresses to facilitate pointing and drainage
- Pain control with acetaminophen, NSAIDs
Recurrent furunculosis (decolonization)
- Intranasal mupirocin 2% ointment BID × 5 days
- Chlorhexidine 4% wash daily for body × 5 days
- Decolonize all household members simultaneously
- Launder linens and clothing in hot water
- Address underlying conditions: diabetes, eczema, HS
Facial / central face (danger triangle)
- Avoid I&D of small lesions in danger triangle (between nose and upper lip) — theoretical risk of septic cavernous sinus thrombosis (rare with antibiotic era but caution remains)
- Treat with antibiotics; I&D only if significant collection
Carbuncle / complex deep abscess
- Surgical incision and debridement under regional anesthesia
- Wider antibiotic coverage including IV options if systemic toxicity
- Address comorbidities (diabetes, immunosuppression)
Second-line / adjunct
- Bleach baths (1/4 to 1/2 cup household bleach in full tub) twice weekly for recurrent disease (especially with atopic dermatitis)
- Hot tub folliculitis (Pseudomonas aeruginosa) — self-limited, supportive care; ciprofloxacin only for immunocompromised or severe
- Gram-negative folliculitis (post-antibiotic acne treatment) — discontinue antibiotic, switch class
- Tinea barbae / sycosis barbae — antifungal not antibacterial therapy
Complications
- Spread to adjacent soft tissue → cellulitis, lymphangitis
- Bacteremia and sepsis
- Endocarditis (especially with IV drug use)
- Osteomyelitis, septic arthritis
- Necrotizing fasciitis (rare but life-threatening)
- Cavernous sinus thrombosis from facial 'danger triangle' infections (very rare with antibiotics)
- Scarring, especially after carbuncle
- Antibiotic adverse effects: C. difficile colitis (clindamycin), photosensitivity (doxycycline), SJS/hyperkalemia (TMP-SMX)
PANCE pearls
- I&D is the primary treatment — antibiotics WITHOUT I&D usually fail.
- TMP-SMX and doxycycline cover MRSA but NOT group A strep — for cellulitis without abscess, use cephalexin or add coverage.
- Bedside ultrasound is invaluable for ambiguous fluctuance — show the patient and document.
- Recurrent furunculosis requires decolonization of the patient AND all household members — intranasal mupirocin + chlorhexidine washes.
- Pain disproportionate to exam, rapid spreading erythema, crepitus, or systemic toxicity = necrotizing fasciitis until proven otherwise — surgical emergency.
References
- IDSA 2014 — Practice Guidelines for the Diagnosis and Management of Skin and Soft Tissue Infections: 2014 Update by the Infectious Diseases Society of America (Stevens et al., Clin Infect Dis 2014)
- Liu CA-MRSA 2011 — Clinical Practice Guidelines by the Infectious Diseases Society of America for the Treatment of MRSA Infections in Adults and Children (Liu et al., Clin Infect Dis 2011)
- CDC MRSA — CDC Healthcare-Associated Infections and MRSA Clinical Guidance
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