Cellulitis
Infectious Disease
Illness script · Infectious Disease
Cellulitis
Acute bacterial infection of the deep dermis and subcutaneous fat, most often caused by Streptococcus pyogenes or Staphylococcus aureus.
This illness script for Cellulitis covers predisposing factors, classic presentation, mechanism, workup, management, and the clinical pivots that separate it from look-alikes—written for USMLE Step 1 and clerkship reasoning.
01
Predisposing factors
- Most common on lower extremities; portal of entry often a skin break, tinea pedis, or ulcer
- Lymphedema, chronic venous insufficiency, and obesity are major risk factors
- Immunocompromised states (diabetes, HIV, cirrhosis) increase risk and severity
- Prior cellulitis is the strongest predictor of recurrence
- Obesity and disrupted skin barrier (eczema, wounds) predispose
02
Presentation
- Non-pitting erythema, warmth, edema, and tenderness — classically unilateral lower leg
- Ill-defined, spreading borders distinguish it from well-demarcated erysipelas
- Systemic signs (fever, chills, malaise) present in moderate-to-severe cases
- Skin may develop bullae or desquamation in severe/progressive disease
- Regional lymphadenopathy and lymphangitic streaking may be seen
- No fluctuance or purulent drainage (if present, think abscess/purulent cellulitis)
03
Pathophysiology
- Bacteria breach the skin barrier and invade the dermis/subcutaneous tissue
- Streptococci spread via lymphatics; staph tends to cause more localized/abscess formation
- Host inflammatory response (cytokines, neutrophils) produces classic erythema, warmth, swelling
- Bacteremia is uncommon but can occur in immunocompromised patients
04
Diagnostics
- Clinical diagnosis — no routine labs or imaging required for uncomplicated cases
- CBC may show leukocytosis; blood cultures rarely positive (~2%) but drawn if septic
- Mark borders with skin marker to monitor progression at 24–48 hours
- Ultrasound to rule out underlying abscess if fluctuance is suspected
- Bilateral lower extremity involvement should prompt alternative diagnosis (stasis dermatitis)
05
Management
- Non-purulent cellulitis: oral cephalexin or dicloxacillin (beta-strep coverage) for 5–7 days
- MRSA risk factors or purulent: add TMP-SMX or doxycycline; or clindamycin
- IV antibiotics (nafcillin, cefazolin, or vancomycin for MRSA) for systemic toxicity or treatment failure
- Elevate affected limb to reduce edema and speed resolution
- Hospitalize if rapidly spreading, hemodynamic instability, immunocompromise, or failed outpatient therapy
06
Clinical pivots
How to separate this script from the look-alikes that show up on exams and on the wards.
Erysipelas
Well-demarcated, raised border with intense erythema; involves superficial dermis/lymphatics; almost always Strep
Necrotizing Fasciitis
Pain out of proportion, crepitus, rapid progression, skin necrosis — surgical emergency requiring immediate debridement
Deep Vein Thrombosis
DVT causes unilateral leg swelling/erythema but lacks warmth spreading from a skin break; duplex ultrasound differentiates
Stasis Dermatitis
Bilateral, chronic, associated with varicose veins and pitting edema; not infectious — often misdiagnosed as cellulitis
Keep reading
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- Community-Acquired PneumoniaAcute lower respiratory tract infection acquired outside hospital, causing alveolar consolidation and impaired gas exchange.
Educational use only. This illness script is a study framework, not medical advice. Confirm decisions with current guidelines and your clinical supervisors.