St Emlyn’s Lesson Plans
The red leg: cellulitis, mimics and dangerous infection
Compare cellulitis, a non-infective red leg and possible necrotising infection, with attention to treatment and reassessment.
Early-career Emergency Medicine · 45-minute session
For learners
Before: Allow about 18 minutes in total.
When we meet: We will compare three red-leg presentations and discuss the diagnosis, treatment setting and advice each person needs.
For facilitators
Send: the learner preparation link before teaching.
Bring: Local antibiotic pathway, pen and paper, and a whiteboard with columns for diagnosis, severity and treatment setting.
Before the session
If you are attending this session, complete the core preparation below before you come. Allow about 18 minutes in total.
Core preparation
NICE NG141: Cellulitis and erysipelas · About 15 minutes
Read NG141’s initial-management and reassessment recommendations, then the adult antibiotic table. Focus on oral versus IV treatment, review timing and signs that should prompt escalation.
https://www.nice.org.uk/guidance/ng141/chapter/Recommendations
Think about (allow 3 minutes): Which finding would make you abandon a routine cellulitis plan and seek urgent surgical help?
Picture the scene
The board says “rash”. The patient has diabetes, fever and a painful leg. What do you need to establish before calling this routine cellulitis?
By the end of the session
- Build a differential for a red, painful or swollen leg using history, examination and systemic features.
- Choose an appropriate treatment setting and antibiotic route using current guidance and local policy.
- Recognise possible deep or necrotising infection and escalate without waiting for a score to be reassuring.
After the session
- Review one red-leg consultation for a documented differential and a specific reassessment plan.
Facilitator guide
Send learners the preparation link several days before teaching. Allow about 18 minutes in total.
Before learners arrive
- Check local antimicrobial and sepsis guidance, ambulatory exclusions and urgent surgical contacts. Decide which details are appropriate to the learners’ prescribing authority.
- Use text cases unless you have appropriately licensed, consented images. Include how assessment differs across skin tones.
Equipment: Local antibiotic pathway, pen and paper, and a whiteboard with columns for diagnosis, severity and treatment setting.
Setup: Ask pairs to compare the three presentations and agree what supports each working diagnosis. Invite a de-identified red-leg case if it adds a useful point, then discuss route, setting and reassessment.
If the preparation has not been completed
Use the five-minute opening to compare three descriptions: a unilateral warm tender leg with fever; longstanding bilateral itchy legs; and rapidly escalating pain with shock. Pairs identify the immediate decision that differs, then check NICE's routine cellulitis recommendations. In the 30-minute plan, use five minutes here and reduce the routine cellulitis case from nine to seven minutes.
Suggested session plan
- 0–5 min — Describe before diagnosing. Ask for the evidence behind the initial label.
- 5–18 min — Routine cellulitis decisions. Explore route and setting, then verify against the actual prescribing pathway.
- 18–28 min — Mimic and explanation. Practise a positive explanation for a non-infective working diagnosis.
- 28–40 min — Dangerous infection. Keep the scenario time pressured enough to expose delays, then discuss how to escalate effectively.
- 40–45 min — Close. Each learner states one finding that would change the plan immediately.
Start the discussion
Read the opening scenario together. Give everyone a moment to think, then ask what concerns them and what they need to know next.
Cases and discussion
Use these cases, or invite an anonymised case from the group that addresses the same questions.
A painful unilateral leg
A 54-year-old with diabetes has a warm, tender patch on one lower leg and fever. They are alert, drinking and haemodynamically stable. There is a small fissure between the toes.
Add this information as the discussion develops:
- Ask learners to describe rather than immediately name the findings.
- The patient asks whether diabetes means they must receive IV antibiotics.
- They can take tablets but struggle to travel daily to hospital.
Ask the group:
- What diagnoses or exposures would alter the usual treatment?
- Which findings support admission, ambulatory care or oral treatment?
- What baseline information makes reassessment meaningful?
- What would make you change the plan over the next two days?
Discussion notes
Points to draw out:
- Exclude mimics and assess severity, comorbidity, site, unusual exposure and ability to take oral medication. Diabetes alone does not automatically determine IV therapy.
- NICE generally favours oral treatment unless oral medicines cannot be taken or illness severity requires IV therapy; review IV treatment by 48 hours when possible. Select the drug, dose and duration using current local policy and NICE, with allergy, renal function and pregnancy checked.
- Mark or document the affected area appropriately, treat predisposing skin problems, address pain and explain the expected course and reassessment triggers. Redness may be less obvious on darker skin: examine warmth, tenderness, swelling and change from baseline.
Common misconceptions:
- Making route depend solely on the diabetes label.
- Calling early persistent redness antibiotic failure without assessing the person and trajectory.
For a more experienced group: Add recent seawater exposure through broken skin. Ask which part of the routine pathway is no longer sufficient.
Bilateral red legs
A 78-year-old has chronic bilateral lower-leg redness, swelling and itching. There is no new systemic illness. Previous courses of antibiotics have made little difference.
Add this information as the discussion develops:
- Ask the group to identify what does not fit ordinary unilateral cellulitis.
- Reveal longstanding venous disease and skin changes; ask how the explanation and follow-up should change.
Ask the group:
- Which alternative causes are plausible?
- How would you explain why another antibiotic may not help?
- What assessment is needed before recommending compression?
Discussion notes
Points to draw out:
- Chronic venous/inflammatory disease is a useful alternative hypothesis, not a diagnosis made from bilateral redness alone. Check for superimposed infection or other acute pathology.
- Arrange appropriate skin/vascular review and practical supportive care. Avoid reproducing a list of specialist drugs or recommending compression without the relevant vascular assessment and local pathway.
Common misconceptions:
- Assuming bilateral disease can never coexist with infection.
- Leaving the person with no explanation because antibiotics were not prescribed.
For a more experienced group: Ask learners to write a two-sentence handover that describes the findings and uncertainty without using “failed antibiotics” as the diagnosis.
Pain and physiology out of proportion
A 42-year-old has rapidly worsening leg pain, fever and hypotension. The skin initially looks less dramatic than the pain would suggest.
Add this information as the discussion develops:
- Ask for the next two minutes of action before offering blood results.
- A colleague proposes waiting for a risk score and CT before contacting surgery.
Ask the group:
- What cannot wait for diagnostic certainty?
- How would you communicate the suspicion to the surgical and critical-care teams?
Discussion notes
Points to draw out:
- Treat this as possible deep/necrotising infection requiring immediate senior, surgical and resuscitation input. Investigations must support, rather than delay, source-control decisions.
- A low laboratory risk score cannot safely exclude necrotising infection when clinical concern persists; do not use it as permission to defer surgical assessment.
Common misconceptions:
- Waiting for gas on imaging or extensive skin necrosis.
For a more experienced group: Rehearse escalation when the initial surgical response is to observe.
Important clinical points
- Use local prescribing guidance for special exposures and infections near the eye or nose.
- Suspected necrotising infection needs immediate surgical assessment; a reassuring score is insufficient.
Closing the session
Use these questions to check what the group will take away. Go back over any clinical points that remain unclear.
- Why might oral therapy be appropriate despite diabetes?
- How would you justify withholding antibiotics while still offering care?
- Which delays are most dangerous in the third case?
Key teaching points:
- Consider non-infective causes as well as cellulitis when assessing a red leg.
- Choose the treatment route and setting from severity, comorbidity and the person’s circumstances.
- Disproportionate pain or deterioration needs prompt reassessment and escalation.
Before leaving, ask each person to choose one action from After the session and say when they will try it. They complete that follow-up in practice, with supervision where needed.
Adapting the discussion
Uncertainty and local practice
Early cellulitis and mimics can overlap. A documented working diagnosis and reassessment plan are more useful than false certainty.
If the discussion drifts
What finding would make your current plan unsafe, and who would you call?
For a more experienced group
Ask the group to compare local ambulatory eligibility with the practical needs of a person who cannot travel daily.
A 30-minute version
30-minute option: introduction 3, cellulitis 9, mimic 6, dangerous infection 9, close 3. Omit unusual-organism detail.
Evidence and guidance
- NICE NG141: Cellulitis and erysipelas — Antimicrobial choice, oral/IV route, reassessment and referral.https://www.nice.org.uk/guidance/ng141/chapter/Recommendations
Suggested curriculum links
RCEM 2021 curriculum (2025 update): SLO1 Adult acute assessment
The cases require differential diagnosis and proportionate management of skin infection and mimics.
Check these links against the learner’s stage and programme. Attendance at a teaching session does not establish competence.
First published 2020-09-17 · Iain Beardsell.