Cellulitis is an acute, spreading, pyogenic inflammation of the lower dermis and subcutaneous tissue.

  • Incidence rising
  • Majority - S. pyogenes or Staph aureus
  • MRSA and VLE versions emerging
  • Dx is clinical
  • Classified using Dundee criteria

Risk factors

  • Venous insufficiency
  • Lymphoedema (also a complication)
  • Peripheral vascular disease
  • Diabetes mellitus
  • Obesity
  • Tinea pedis
  • Local ulcers, trauma, insect bites


  • Clinical
  • Usually lower limb
  • Acute, tender, erythematous, swollen area
  • ± ill-distinct margin (subcut. rather than skin infection)
  • ± blisters
  • ± ulcers
  • ± oedema
  • Classically ascending lymphangitis, regional adenitis
  • Fever, Malaise
  • Beware sepsis (See SIRS)
  • Mostly Strep but if pus, most are S. aureus (incl. MRSA)
  • Those due to penetrating trauma or in immunocompromised may be S. aureus or P aeruginosa

Differential Dx

Treatment recommendations for cellulitis based on organisms

Absence of pain or fever; circumferential; bilateral
Involvement of joint; pain on movement
IBD Hx, Local ulceration
/drug rxn
Hx exposure, pruritus, not toxic, little pain
DVT Low (if any) fever, few skin changes
Severe pain, swelling & fever; rapid progression; pain out of proportion; systemic toxicity; skin crepitus; necrosis; ecchymosis


  • Check blood glucose in all
  • WCC / CRP / d-dimers are not helpful
  • U&E / Creat in all
  • Blood cultures if SIRS (incl. >38°C), animal bite, ocular/facial infection, immersion or immunocompromised
  • Swab (C&S) open wounds
  • Imaging if suspected DVT or abscess/ostemyelitis
  • Surgery now if suspicion of necrotising facsiitis


  • Analgesia
  • Treat the patient, not the test
  • Rest, elevation, analgesia ± VTE thromboprophylaxis
  • Mark the area
  • IV or PO antibiotics
  • Beware SIRS
  • Beware possibility MRSA or VLE in susceptible patients
  • Admit CDU if IV antibiotics required and no SIRS
  • Out patient IV antibiotics only after discharge from CDU

Atypical Scenarios

Clinical Organism Antibiotic (nchd.ie)
Typical cellulitis Strep. pyogenes Clindamycin or Amoxicillin or Flucloxacillin
Typical cellulitis—pus forming Staph. aureus Clindamycin or Flucloxacillin
Penicillin allergy NA Clindamycin or Clarithromycin
Cat or dog bite Pasteurella multocida Co-amoxiclav; if allergic to penicillin: doxycycline and metronidazole
Freshwater exposure Aeromonas hydrophila Ciprofloxacillin
Salt-water exposure Vibrio vulnificus Doxycycline
Necrotising fasciitis Clostridium perfringens Benzylpenicillin, ciprofloxacillin, and clindamycin
Butchers and fish handlers Erysipelothrix Ciprofloxacillin

Dundee classification of severity

Patients should be stratified into four classes of severity (class IV most severe) based

  1. Sepsis
  2. Co-morbidity
  3. Standardized early warning score (SEWS)

SIRS = ≥2 of: <4 WCC >12, <36° Temp >.388C, HR > 90, RR >20

Further details to follow - but class 3 (significant co-morbidities or SIRS features are not suitable for CDU).


For CDU patients please start either:

  1. Cefazolin 2g q24h iv plus probenicid 1g po q24h if CIT suitable or
  2. Flucloxacilin (± penicillin) (more on nchd.ie)

Cefazolin / Probenicid

More details on home IV antibiotics for cellulitis.

If the patient is unable to tolerate the once daily regimen due to GI upset, then prescribe Cefazolin 2g twice daily iv without the probenacid.

Check dosing (with pharmacist) in renal impairment.

For penicillin allergic (OPAT/CIT only) patients, please contact micro. for advice on vancomycin dosing and monitoring.

Probenecid contra-indicated with:

  • Methotrexate
  • Sodium phenylbutyrate
  • Ketorolac
  • Pyrazinamide
  • High dose aspirin
  • Dapsone
  • Heparin
  • Fosfomycin

More details on home IV antibiotics for cellulitis.

Content by Dr Íomhar O' Sullivan. Last review Dr ÍOS 27/05/23.