Clinical meaning
Cellulitis is an acute, spreading bacterial infection of the dermis and subcutaneous tissue, most commonly caused by beta-hemolytic streptococci (Group A Strep, S. pyogenes) and Staphylococcus aureus (including MRSA). Bacteria enter through breaks in the skin barrier (trauma, surgical wounds, tinea pedis, insect bites, venous stasis ulcers) and spread laterally through tissue planes via hyaluronidase and other enzymes that degrade extracellular matrix. The infection triggers an intense inflammatory response with neutrophil infiltration, vasodilation, and increased capillary permeability, producing the cardinal signs of erythema, warmth, swelling, and tenderness. Unlike erysipelas (which involves the upper dermis and has sharply demarcated, raised borders), cellulitis extends deeper into subcutaneous tissue and has indistinct, non-elevated borders. Purulent cellulitis (with abscess or drainage) is more likely MRSA; non-purulent cellulitis is more likely streptococcal.
Diagnosis & workup
Diagnostics & workup: - Clinical diagnosis based on the 4 cardinal signs: erythema, warmth, swelling, and tenderness with indistinct borders - Mark the leading edge of erythema with a skin marker and timestamp to objectively track progression or improvement - Blood cultures: obtain only if systemic toxicity (fever, tachycardia, hypotension) or immunocompromised; positive in only ~2% of uncomplicated cases - CBC with differential: leukocytosis with left shift supports bacterial infection - CRP/ESR: elevated in cellulitis; useful for monitoring treatment response - Wound culture: only if purulent drainage, abscess, or open wound present; NOT from intact skin surface - Ultrasound: to evaluate for underlying abscess (fluid collection requiring I&D) that may not be clinically apparent - Consider imaging (CT or MRI) if necrotizing fasciitis is suspected (rapidly advancing, pain out of proportion, crepitus, systemic toxicity)