Estimated Reading Time: 5 minutes

Picture the scene…

You are working in minors where you are about to see a patient with “a rash”…

Learning Objective

To learn about the diagnosis and treatment of Cellulitis and its differential diagnoses.

RCEM Curriculum

CAP28 – Rash

Read this blog post from RCEM Learning about the diagnosis of cellulitis and other skin diseases.

Listen to this podcast from Rob Orman at ER Cast, featuring excerpts of a lecture from Greg Moran at Essentials of EM.

This part of the teaching session should be lead by an experienced clinican. It is especially important in this lesson to highlight pathways that may be used in your hospital. The cases provided are merely examples and if possible the learners should be encouraged to discuss patients they have seen in their clinical practice.

A 54 year old with diabetes presents to the ED feeling unwell with a rash on their leg. They have a fever and feel cold all the time. The rash is warm and painful to touch.

Image from

This is cellulitis. A bacterial infection of the skin that is usually unilateral and localised, but often with systemic symptoms. It affects the dermis and subcutaneous tissue.

Streptococcus pyogenes (2/3) and Staphylococcus aureus (1/3) are the most common bacterial pathogens.

1, Analgesia – Cellulitis can be really painful, so don’t forget pain relief

2, Antibiotics – In this patient with co morbodities (diabetes) and systemic symptoms intravenous therapy may be appropriate. Many hospitals may offer this on an “ambulatory” basis – ie the patient does not need to be admitted, but attends daily for antibiotics. The choice of antibiotic will depend on local guidelines and sensitivities. Usually penicillin based agents are chosen, although single daily dose cephalosporins may aid with outpatient therapy.

3, Rest and elevate

4, Treat the underlying cause if there is one.

  • Pseudomonas aeruginosa, usually in a puncture wound of foot or hand
  • Haemophilus influenzae, in children with facial cellulitis
  • Anaerobes, EikenellaStreptococcus viridans, due to human bite
  • Pasteurella multocida, due to cat or dog bite
  • Vibrio vulnificus, due to saltwater exposure, eg coral injury
  • Aeromonas hydrophila from fresh or saltwater exposure, eg following leech bites
  • Erysipelothrix (erysipeloid), in butchers.

If these are suspected the antibiotic choice is crucial as they may not respond to the usual treatments.

A 78 year old presents to the ED with bilateral rednesson their legs. They are generally well, with no fever.

Image from

This is lipodermatosclerosis and not cellulitis. It is a chromic inflammatory condition and due to subcuteneous hardening of the skin.

In some ways it can mimic cellulitis, but is often bilateral and does not cause systemic symptoms.

Exercise may help with both weight reduction (which can be a predisposing factor) and venous insufficiency (via the muscle pump).

Compression (with compression stockings) and elevation may aid with venous drainage.

Medical treatments such as Stanozolol, Pentoxifylline, Hydroxychloroquine, Intralesional steroid injection of triamcinolone and Tetracyclines may be used, but would be outside the remit of the emergency physician. Surgery may be indicated to treat underlying venous insufficiency.

If a patient presents unwell with signs of infection then this leg rash is unlikely to be the cause and another source must be sought.

If there has been extension of the rash in the preceeding few days or there is new or worsening pain over the rash this could be superimposed infection and may be the source of the sepsis.

A 42 year old presents with fever, tachycardia and hypotension with a very painful rash on their lower leg.

Image from

This patient has sepsis and needs urgent treatment. They have a low blood pressure and tachycardia and should be treatment in a high dependancy area such as Resus.

You should alert a senior colleague and start urgent fluid therapy. Broad spectrum antibiotics should be given intravenously.

This is necrotising fasciitis until proven otherwise,

Infection starts in the superficial fascia, often after minor trauma. Enzymes and proteins released by the responsible organisms cause necrosis of fascial layers.

It may be several days before the infection spreads vertically onto the skin, so patients can present late.

Thrombosis occurs as a result of the toxins released by the bacteria and may cause ischaemia and subsequent necrosis of the tissues.

Radiology – Xray, CT or MRI may all confirm the presence of gas and inflammation in the soft tissues.

The LRINEC score can be used although if the clincal suspicion is high then this score should not overrule decision making. It uses CRP, WCC, Hb, Sodium, Creatinine and Glucose.

This is a potentially life threatening condition and needs prompt identification and treatment. They can become very unwell very quickly.

Many of these patients will need intensive care involvement and admission to a critical care unit.

The patient may need surgical debridement to enable source control.

Identification of the causative organism is key to targetting treatment.

In this session we have learned about the clinical assessment of the patient with suspected cellulitis, and several of its mimics. Not all rashes are cellulitis and can range from chronic inflammation to life threatening sepsis.

In order to embed today’s learning further, reflect on what you have learnt and record in your portfolio whether it has had any impact (or is expected to have any impact) on your performance and practice.

Was this a topic that you were confident you knew already? Which parts were new to you? Were there elements that you will use on your next clinical shift.

Dscuss this session with your colleagues – were there people who missed it who you can share the highlights with?

References and Further Reading

1, Helen Yasmin Sultan. Cellulitis. RCEM Learning. Published online 20th October 2017. (accessed 17th September 2020).

2, Rob Orman. Cellulitis. ERCast. Published online 29th January 2018. (accessed 15th September 2020)

DermNetNZ is a superb FOAMed website that has all of the information you could possibly require about skin conditions.