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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Question 1DermatologyMedium
A 32-year-old man has a painful fluctuant erythematous nodule on the posterior thigh. There is no surrounding cellulitis or systemic toxicity. Which of the following is the most appropriate initial treatment?
AWarm compresses and observation
BIncision and drainage of the abscess
CNeedle aspiration of the abscess
DOral cephalexin monotherapy
EOral trimethoprim-sulfamethoxazole
Reveal answer & full explanation
Correct answer: B — Incision and drainage of the abscess
AWarm compresses and observation
BIncision and drainage of the abscess✓
CNeedle aspiration of the abscess
DOral cephalexin monotherapy
EOral trimethoprim-sulfamethoxazole
Why Incision and drainage of the abscess is correct
A painful fluctuant nodule is a cutaneous abscess, and source control by incision and drainage is the primary therapy.
Fluctuance signals a walled-off purulent collection that antibiotics alone cannot clear.
Systemic antibiotics are reserved for surrounding cellulitis, systemic signs, immunocompromise, or high-risk sites.
Why the others are wrong
Needle aspiration of the abscess — Near-miss trap: aspiration often leaves pus behind and has lower cure rates than incision and drainage, especially for MRSA abscesses, so it is not recommended as primary therapy.
Warm compresses and observation — Delay trap: a small early furuncle may resolve with heat, but an established fluctuant abscess needs drainage.
Oral trimethoprim-sulfamethoxazole — Right-drug-wrong-step: TMP-SMX covers community-acquired MRSA and may be added after drainage, but an antibiotic without source control cannot clear a walled-off fluctuant collection.
Oral cephalexin monotherapy — wrong-drug-wrong-step trap: cephalexin is a standard oral choice for nonpurulent cellulitis, which is usually streptococcal, but this fluctuant abscess is a purulent infection most often caused by S. aureus, frequently community-acquired MRSA, which cephalexin does not cover, and no antibiotic substitutes for drainage of the collection.
Question 2DermatologyMedium
A 34-year-old man presents with 4 days of a painful, enlarging red lump on his left thigh. On examination there is a 4-cm tender, warm, erythematous area of induration, but the clinician cannot clearly determine whether a drainable fluid collection is present versus diffuse inflammation alone. He is afebrile with normal vital signs and no surrounding lymphangitis. Which of the following is the most appropriate next diagnostic test?
AContrast-enhanced CT of the thigh
BVenous duplex ultrasound of the leg
CWound culture of the swelling
DBedside soft-tissue ultrasound
EPlain radiograph of the thigh
Reveal answer & full explanation
Correct answer: D — Bedside soft-tissue ultrasound
AContrast-enhanced CT of the thigh
BVenous duplex ultrasound of the leg
CWound culture of the swelling
DBedside soft-tissue ultrasound✓
EPlain radiograph of the thigh
Why Bedside soft-tissue ultrasound is correct
When the clinical exam cannot reliably distinguish a fluctuant abscess (which needs incision and drainage) from cellulitis alone (which does not), point-of-care ultrasound is the test of choice. It identifies an anechoic or hypoechoic fluid collection and reveals loculations.
It is fast, radiation-free, inexpensive, and changes management directly: a discrete collection prompts I&D, whereas its absence supports treating as cellulitis.
Why the others are wrong
Wound culture of the swelling is obtained from purulent material after drainage and only when the infection is severe, recurrent, immunocompromised, or treatment-failure; it cannot establish whether a drainable collection exists.
Contrast-enhanced CT of the thigh is reserved for deep, complex, or perirectal abscesses or suspected necrotizing fasciitis; it is excessive for a superficial extremity lesion that ultrasound resolves at the bedside.
Plain radiograph of the thigh poorly characterizes fluid collections and is used mainly to look for gas or foreign bodies, not to confirm a routine abscess.
Venous duplex ultrasound of the leg — a warm, red, tender swelling of the leg often prompts a DVT workup, but DVT typically causes diffuse limb swelling rather than a focal 4-cm indurated lump, and a venous duplex study is aimed at compressing the deep veins, not at deciding whether this lump contains a drainable collection.
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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; rose thorns or gardening (Sporothrix), fish tanks or aquariums (M. marinum)
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]
Cutaneous abscess and furuncle: I&D-centered management algorithm.
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 bases the decision on SIRS signs, failed I&D or impaired host defenses; later RCTs, Talan NEJM 2016 and Daum NEJM 2017, showed higher cure rates with TMP-SMX or clindamycin even for uncomplicated abscesses):
• 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)
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
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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