Acute bacterial infection of the dermis and subcutaneous tissue, most often caused by beta-hemolytic streptococci or Staphylococcus aureus.
Also known as: cellulitis, skin infection, soft tissue infection, SSTI
Overview
Non-purulent or purulent bacterial infection of the deeper dermis and subcutaneous fat, characterized by warm, erythematous, edematous, tender skin without sharp borders. Distinguished from erysipelas (more superficial, sharply demarcated) and necrotizing fasciitis (deep, rapidly progressive, systemic toxicity).
Epidemiology
Among the most common reasons for emergency visits and hospitalization for skin and soft tissue infection. Annual incidence ~200 per 100,000. Recurrence common (~30% within 3 years) especially in patients with lymphedema or prior cellulitis.
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Question 1Infectious DiseaseMedium
A 48-year-old man has a warm, tender, expanding erythematous plaque on the lower leg with fever. There is no fluctuance or purulent drainage. Which of the following is the most likely diagnosis?
AAcute stasis dermatitis
BNonpurulent cellulitis
CDeep venous thrombosis
DDrainable cutaneous abscess
Reveal answer & full explanation
Correct answer: B — Nonpurulent cellulitis
AAcute stasis dermatitis
BNonpurulent cellulitis✓
CDeep venous thrombosis
DDrainable cutaneous abscess
Why Nonpurulent cellulitis is correct
Spreading warmth, erythema, tenderness, and fever without fluctuance or pus is the textbook picture of nonpurulent cellulitis.
It is usually caused by beta-hemolytic streptococci, which is why coverage targets streptococcal organisms.
The absence of a drainable collection distinguishes it from purulent skin infection and guides antibiotic choice.
Why the others are wrong
Drainable cutaneous abscess — Purulence trap: an abscess requires fluctuance or a localized pus pocket, both explicitly absent here.
Acute stasis dermatitis — Mimic trap: stasis dermatitis is usually bilateral, chronic, and itchy rather than a unilateral acutely expanding tender plaque with fever.
Deep venous thrombosis — Anchoring trap: DVT causes a swollen tender leg but lacks the well-demarcated warm erythematous plaque and fever of cellulitis.
Question 2Infectious DiseaseMedium
A 44-year-old man with type 2 diabetes presents with 3 days of a warm, red, tender area on his right forearm at the site of an old IV catheter. On exam there is poorly demarcated erythema, warmth, and a focal area of fluctuance with surrounding induration. He is afebrile and hemodynamically stable, with no pain out of proportion, crepitus, or systemic toxicity. You suspect cellulitis but need to determine whether a drainable fluid collection is present before deciding between antibiotics alone and incision and drainage. Which of the following is the most appropriate next diagnostic test?
APlain radiography of the forearm
BAerobic and anaerobic blood cultures
CBedside soft-tissue ultrasonography
DContrast-enhanced CT of the forearm
Reveal answer & full explanation
Correct answer: C — Bedside soft-tissue ultrasonography
APlain radiography of the forearm
BAerobic and anaerobic blood cultures
CBedside soft-tissue ultrasonography✓
DContrast-enhanced CT of the forearm
Why Bedside soft-tissue ultrasonography is correct
Point-of-care ultrasound is the first-line study to distinguish uncomplicated cellulitis (subcutaneous "cobblestoning" from edema) from a drainable abscess (anechoic or hypoechoic fluid collection).
A fluctuant focus signals a possible abscess, and confirming a collection changes management from antibiotics alone to incision and drainage.
It is rapid, radiation-free, and bedside-available, making it the appropriate next step when an abscess is in question.
Why the others are wrong
Contrast-enhanced CT of the forearm is used when necrotizing fasciitis or deep/osteomyelitic spread is suspected; this stable, afebrile patient has no pain out of proportion, crepitus, or systemic toxicity, so CT adds cost and radiation without being first-line for simple abscess detection.
Plain radiography of the forearm is reserved for retained foreign body, soft-tissue gas, or osteomyelitis and cannot reliably identify a soft-tissue fluid collection.
Aerobic and anaerobic blood cultures are recommended only with severe sepsis, immunocompromise, water exposure, or bite wounds; in mild localized infection they rarely change management and do not localize an abscess.
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Diabetes mellitus (especially with peripheral neuropathy)
Immunocompromise
Pathophysiology
Bacteria enter through breaks in the skin and proliferate in the dermal/subcutaneous space. Streptococcal cellulitis spreads rapidly via hyaluronidase and streptokinase, often without purulence. Staphylococcal infection (including MRSA) more often produces abscess and purulent drainage. Systemic toxicity reflects cytokine release and bacteremia in severe cases.
Clinical presentation
Symptoms
Localized warmth, erythema, edema, tenderness — usually a single extremity
Spreading borders without sharp demarcation
Fever, chills, malaise in moderate/severe disease
Possible regional lymphangitis ('red streaking') and lymphadenopathy
Signs / physical exam
Warm, indurated, erythematous skin with poorly defined edges
Possible purulent drainage from puncture site or abscess
Lymphangitic streaking toward draining nodes
Look for portal of entry (tinea pedis, ulceration, IV site)
Classic findings
Unilateral lower-leg cellulitis is extremely common; bilateral lower-leg 'cellulitis' is almost never cellulitis — consider stasis dermatitis, lipodermatosclerosis, or contact dermatitis.
Differential diagnosis
DVT — Unilateral leg swelling and pain, calf tenderness, Homans sign — but erythema usually less prominent; duplex ultrasound
Stasis dermatitis — Bilateral, chronic, hemosiderin staining, weeping/crusting; not warm or tender; often misdiagnosed as cellulitis
Contact or allergic dermatitis — Pruritic > tender, vesicular or scaling, sharp borders matching exposure
Necrotizing fasciitis — Pain out of proportion, rapid progression, bullae, crepitus, systemic toxicity; surgical emergency
Erysipelas — Bright red, sharply raised borders ('orange peel'), more superficial; usually streptococcal
Endocarditis (especially with bacteremic Staphylococcus aureus)
Osteomyelitis (especially diabetic foot)
PANCE pearls
Treat predisposing tinea pedis — interdigital fissures are a recurrent portal of entry; eradicating the entry portal reduces recurrence (note: the PATCH trials demonstrated benefit from prophylactic penicillin, not antifungals).
Bilateral lower-extremity 'cellulitis' is rare; reconsider the diagnosis (stasis dermatitis is the great mimicker).
Cellulitis often appears worse on day 1-2 of antibiotics as bacterial lysis releases mediators — do not switch antibiotics for warmth/redness alone in the first 48 hours.
Outline the erythema with a marker on admission to monitor progression objectively.
Severe pain out of proportion, bullae, crepitus, or systemic toxicity → necrotizing fasciitis; immediate surgical consult.
References
IDSA 2014 — Stevens et al., Practice Guidelines for the Diagnosis and Management of Skin and Soft Tissue Infections (Clin Infect Dis)
PATCH I/II — Thomas et al., Penicillin to prevent recurrent leg cellulitis (NEJM 2013)
Aboltins 2014/Hepburn — Trials supporting 5-7 day antibiotic courses for uncomplicated cellulitis
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