Cellulitis & Skin/Soft Tissue Infections

You draw a line around the redness with a skin marker at 2 PM — by 6 PM it's an inch past the line. That marker just told you the antibiotics aren't working before any lab result could.

Cellulitis is a non-purulent, spreading bacterial infection of the skin and subcutaneous tissue, most commonly caused by Group A Streptococcus or Staphylococcus aureus. It presents with unilateral erythema, warmth, swelling, tenderness, and poorly defined borders, most frequently on the lower extremities. Risk factors: skin breaks (trauma, ulcers, tinea pedis, insect bites), lymphedema, chronic venous insufficiency, obesity, diabetes, and immunosuppression. A critical nursing intervention is marking the borders of erythema with a skin marker and noting the time — this objectively tracks whether the infection is spreading or responding to treatment. Uncomplicated nonpurulent cellulitis: cephalexin or dicloxacillin for streptococcal/MSSA coverage. If MRSA risk is present, add MRSA-active therapy such as TMP-SMX or doxycycline plus beta-lactam streptococcal coverage, or use clindamycin when appropriate. Systemic signs (fever >38°C, tachycardia, hypotension, rapidly spreading erythema): IV antibiotics and possible hospitalization. Elevate the affected extremity to reduce edema and promote drainage. Abscess: fluctuant, localized collection of pus — requires incision and drainage (I&D) as primary treatment; antibiotics alone are insufficient. Culture the drainage. Necrotizing fasciitis is the critical emergency differential: suspect when pain is severe and disproportionate to the visible skin findings, with rapid progression, crepitus on palpation (gas in tissues), dusky or necrotic skin, and systemic toxicity. This requires emergent surgical debridement — antibiotics alone will not save the patient. Nursing: monitor for spreading (reassess margins every 4–8 hours), assess for abscess development (fluctuance), teach wound care and prevention (treat tinea pedis, moisturize dry cracked skin, wear protective footwear), and assess for DVT in unilateral lower extremity swelling (different mechanism but similar appearance). Prioritize elevation and provider-directed therapy; avoid unsupervised heat application if swelling or pain worsens or vascular compromise is a concern.

Key Distinctions

Pain out of proportion to visible findings is the hallmark of necrotizing fasciitis — this is a surgical emergency, not an antibiotic problem. Students confuse cellulitis (spreading, non-purulent, no defined border) with abscess (localized, fluctuant, purulent — needs I&D). When MRSA is suspected in nonpurulent cellulitis, TMP-SMX or doxycycline alone may not cover streptococci adequately — pair with a beta-lactam or consider clindamycin. Cellulitis can mimic DVT in the lower extremity — both cause unilateral redness, swelling, and pain, but DVT typically has deeper calf pain and may lack the warmth and skin changes of cellulitis.

Clinical Pearl

Draw a line, write the time. If the redness crosses it, the treatment isn't working — escalate before the labs catch up.