CELLULITIS

✓ Medically ReviewedDISEASEID: MDBT0142🚨 TRIAGE: URGENTICD-10: L03.90

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Definition

CLINICAL DEFINITION

Cellulitis is an acute bacterial infection of the dermis (deep skin layer) and subcutis (fat under skin). It causes spreading redness (erythema), warmth, swelling and pain. Fever with red streaks (lymphangitis along lymph vessels) is common. The process is usually non-necrotising (without tissue death). Diagnosis is clinical after excluding necrotising infection and mimics.

IN PLAIN ENGLISH

Germs get into skin through a cut. Skin turns red, warm, swollen and sore. Leg is the most common site. Antibiotics treat most cases. Fast care stops spread to blood.

COMMON MISCONCEPTION / WHAT IT DOES NOT MEAN

Cellulitis is not cellulite (cosmetic dimpled fat without infection). It is not erysipelas alone (sharply raised superficial infection) though overlap exists. It is not necrotising fasciitis (rapid deep tissue death with severe pain). It is not stasis dermatitis (chronic redness from vein disease) or deep vein thrombosis (leg blood clot). Any red leg without warmth, pain or fever is not cellulitis.

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Word Breakdown

cellul

=small cell or tissue space from Latin cellula

-itis

=inflammation meaning immune response with swelling

Literal Meaning: inflammation of small tissue compartments

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Abbreviations

SSTI=Skin and soft-tissue infection - umbrella term for bacterial skin infections

ABSSSI=Acute bacterial skin and skin-structure infection - regulatory term for severe trial cases

MRSA=Methicillin-resistant Staphylococcus aureus - drug-resistant staph needing special antibiotics

GAS=Group A Streptococcus - Streptococcus pyogenes (common streptococcal cause)

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Synonyms & Variants

Bacterial cellulitis›Acute bacterial skin infection›Dermohypodermitis›

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Category-Specific Context

Cellulitis is a Disease because it is a distinct infective process with causes, course and treatment. It is not a single Symptom (patient complaint) or Sign (exam finding). It produces a syndrome (linked pattern) of redness, warmth, swelling, pain and fever. It needs disease-level antibiotics with source control and follow-up. It predicts abscess, blood spread and recurrence.

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Clinical Context & Usage

CLINICAL SETTINGS

Primary Care for Spreading Red LegEmergency Department for Fever with RednessUrgent Care ClinicDermatology ClinicInfectious Disease ServiceDiabetic Foot Clinic

USAGE & DOCUMENTATION NOTES

Use cellulitis for acute spreading bacterial infection of skin and under-skin fat with indistinct borders. Document site, extent, pain, warmth, swelling and systemic features. State purulent (pus present) versus non-purulent (no pus) with severity. Avoid labelling chronic red legs without warmth, pain or fever. Mark the leading edge and reassess within 24 to 48 hours.

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Primary Causes & Pathophysiology

PATHOPHYSIOLOGY

Bacteria enter through a break in the epidermis (outer skin barrier). Triggers include cuts, ulcers (open sores), cracks and tinea pedis (athlete foot fungal infection). They spread in dermis (deep skin) and subcutis (under-skin fat). Toxins activate innate immunity (early infection defence). Vessel widening with neutrophil influx (infection-fighting white cells) causes redness, warmth and swelling. Lymph spread causes red streaks and can progress to pus or blood infection.

PRIMARY CAUSES

Streptococcus pyogenes and other beta-haemolytic streptococci

Adhere to damaged skin then spread through lymph channels and trigger diffuse redness with fever

Staphylococcus aureus including sensitive and resistant strains

Colonise skin and wounds then release toxins and enzymes that cause pus-forming inflammation with abscess tendency

Skin barrier break from trauma, ulcer, eczema or tinea pedis

Provides an entry point for bacteria into deep skin and under-skin fat

Chronic swelling from venous insufficiency or lymphoedema

Impairs local defence and lymph drainage (fluid clearing) allowing bacterial growth and recurrence

Diabetes, obesity and immunosuppression

Reduce white-cell function and wound healing (repair) raising susceptibility and severity

Bite or water-exposed wounds including Pasteurella, Capnocytophaga, Vibrio and Aeromonas

Introduce aggressive organisms directly into tissue causing rapid infection after specific exposures

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Epidemiology

Prevalence:Cellulitis is common in primary, urgent and emergency care. Risk rises with age and with obesity (excess body fat). Diabetes (high blood sugar disease), venous disease (poor leg vein return) and lymphoedema (lymph swelling) raise risk further. Recurrence is common when leg swelling or ulcers persist.

Age Group:All ages can be affected including children and older adults. Incidence climbs with age and coexisting illness. Young children can develop facial disease after trauma or ear infection. Adults most often develop lower limb disease. Older adults with ulcers, swelling or diabetes carry highest risk.

Geographic Distribution:Cellulitis occurs worldwide in all regions and climates. Cause mix varies by setting and local exposures. No climate restriction applies.

Red Flags & Triage ★

TRIAGE LEVEL

🚨 URGENT

DO NOT MISS — LIFE-THREATENING PRESENTATIONS

Necrotising fasciitis with severe pain and blisters
Orbital cellulitis with eye pain and vision change
Sepsis with low pressure and confusion
Deep vein thrombosis mimicking or coexisting
Diabetic foot osteomyelitis under ulcer
Bite-related Pasteurella or Capnocytophaga in weak immunity

URGENT EVALUATION TRIGGERS

Severe or disproportionate pain with rapidly spreading redness
Blisters, bruise-like change, gas crackling or numb skin
Fever with fast heart rate, low pressure or confusion
Eye pain, double vision or vision change suggesting deep eye spread
Weak immunity, diabetes with foot ulcer or animal bite with systemic upset
No improvement within 24 to 48 hours despite oral antibiotics

Contraindications & Precautions

CONTRAINDICATIONS

Do not incise (cut open) simple non-purulent cellulitis without fluctuance (fluid wave suggesting pus)
Do not delay antibiotics to await extensive testing in typical moderate disease
Do not use antibiotics without checking allergy and interaction history

SPECIAL PRECAUTIONS

Always exclude necrotising fasciitis (deep tissue-killing infection) with severe pain, blisters or gas crackling
Mark redness margins with limb elevation and fever monitoring
Assess diabetes, weak immunity, vein disease and bite or water exposure
Arrange prompt review if spread continues despite antibiotics

Complications

DOWNSTREAM CLINICAL COMPLICATIONS

Abscess (walled pus collection) with fluctuance (fluid wave)
Bacteraemia (bacteria in blood) with sepsis (life-threatening infection response)
Lymphangitis (red streak vessel infection) with lymph node inflammation
Necrotising fasciitis (deep tissue-killing infection) in severe cases
Chronic lymphoedema (persistent lymph swelling) with recurrent episodes
Osteomyelitis (bone infection) under diabetic foot ulcer
Thrombophlebitis (vein inflammation with clot)

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Confused With (LASA & DDx) ★

LEXICAL (LASA) — SIMILAR NAMES

Cellulite

Differs by one letter but means cosmetic dimpled fat (noninfective texture change), not bacterial infection

Panniculitis

Shares itis ending with red nodules but means inflammation of fat lobules (deep fat layer), often nodular rather than spreading sheet

Erysipelas

Shares redness and streptococcal cause but means sharply raised superficial infection, while cellulitis has indistinct deeper borders

CLINICAL DIFFERENTIAL DIAGNOSES (DDx)

Erysipelas

Key Distinguishing Feature: Sharply demarcated raised salmon-red plaque (clear edge) with fever versus indistinct flat border

Necrotising fasciitis

Key Distinguishing Feature: Severe pain out of proportion with blisters, bruise change, gas crackling and toxicity

Deep vein thrombosis

Key Distinguishing Feature: One-sided swelling and ache with minimal warmth and no fever with clot on vein scan

Stasis dermatitis

Key Distinguishing Feature: Chronic bilateral scaly brownish itchy change from vein disease without fever or acute pain

Contact dermatitis

Key Distinguishing Feature: Itchy inflamed skin with exposure history and sharp geometric borders without fever

Cutaneous abscess

Key Distinguishing Feature: Fluid-wave tender pus pocket with central pointing versus diffuse spread

Acute gout

Key Distinguishing Feature: Abrupt single-joint exquisitely tender arthritis (joint inflammation) rather than diffuse skin sheet

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Clinical Pearls

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Mark and date the redness edge and recheck often because spread despite antibiotics suggests pus or tissue-killing infection.

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Examine toe webs for athlete foot cracks and treat this portal because missed fungus drives recurrent leg infection.

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Consider leg blood clot in one-sided swollen painful leg because it can mimic and coexist with infection so use vein scan when unsure.

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Elevate the limb and address swelling and vein disease because dependent swelling slows healing and promotes relapse.

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Take blood cultures before antibiotics in feverish or immunocompromised patients but do not delay antibiotics for results in typical cases.

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Example Usage & SOAP

CLINICAL SENTENCE

""Left shin shows spreading warm tender redness with fever most consistent with cellulitis.""

IN A SOAP NOTE

S: 58-year-old with 2-day left shin pain, redness and swelling after graze with fever and chills, no pus. O: Temp 38.4C, BP 128/78, HR 102, RR 18, SpO2 98 percent on air, left shin diffuse warm tender redness with indistinct edge and swelling, no fluid wave, no gas crackling, pulses present. A: Non-purulent left lower limb cellulitis, moderate, no necrotising signs. P: Mark margins with elevation, blood cultures, start oral beta-lactam per local protocol, analgesia, tetanus check, review in 24 to 48 hours with safety net.

DIFFERENTIAL DIAGNOSIS CONTEXT

In a hot swollen leg contrast cellulitis with erysipelas (sharply raised edge) and necrotising fasciitis (pain out of proportion with blisters). Fever with tender spreading redness and indistinct borders favours cellulitis. One-sided swelling without fever favours clot. Lack of improvement prompts pus search and vein scan.

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Classifications & Scoring

GRADING SYSTEM

Eron classification

Grades cellulitis by systemic features and comorbidity (mild to severe) to guide oral versus intravenous therapy

GRADING SYSTEM

Dundee classification

Stratifies skin infection by extent and systemic upset to guide treatment setting and follow-up

SCORING TOOL

ALT-70 score

Predicts cellulitis versus pseudocellulitis (mimic red leg) using asymmetry, high white cells, fast heart rate and age 70 or older

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Prognosis

SHORT-TERM OUTCOME

Most non-purulent (no pus) cases improve within days on suitable oral antibiotics. Pain, fever and redness usually ease within a day or two with rest and elevation. Delay raises risk of pus, vessel streak infection or blood spread. Rapid pain with blisters or low pressure signals possible necrotising infection needing emergency surgery.

LONG-TERM PROGNOSIS

Most episodes resolve without lasting change when treated promptly. Recurrent leg disease can cause chronic lymph swelling with scar thickening. Persistent vein disease, athlete foot or ulceration predicts relapse. Control of swelling, skin care and weight lowers recurrence.

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References & Coding

ICD-10 DIAGNOSTIC CODE

L03.90

LITERATURE CITATIONS & GUIDELINES

[1]WHO ICD-10 L03 Cellulitis

[2]IDSA skin and soft-tissue infections guidance

[3]NICE guidance on cellulitis and erysipelas

[4]Harrison's Principles of Internal Medicine skin infections

MEDICAL DISCLAIMER — This content is intended for educational purposes and qualified healthcare professionals only. Always refer to current clinical diagnostic criteria, institutional protocols, and official guidelines. Medobo is not responsible for clinical decisions made based on this content.