Cellulitis
Usually one leg. Draw the line, find the entry point and judge severity. Two red legs are rarely cellulitis.
A 55-year-old diabetic woman has a red, hot, swollen left shin for 3 days. Cracked skin between her toes. Temperature 38.2 °C, pulse 96, BP 132/80. Glucose 13.4 mmol/L (241 mg/dL).
Oral or IV, and what else do you check?
- Look for danger firstSevere pain, blisters, dusky skin, crackling, rapid spread, septic: think necrotising fasciitis.
- Mark the edgePen line, date and time. You will judge response by it.
- Find the entry pointCracks between toes, wounds, ulcers, insect bites, injection sites.
- Grade severityEron class decides oral or IV, home or admit.
- Antibiotic and elevationLeg up above the hip.
- Treat the causeAthlete's foot, oedema, sugar control.
- Review in 48 hoursRedness may look worse on day 1 even when working.
- Pain far worse than the redness looks
- Blisters with dusky or purple skin, crackling under the skin
- Septic or low BP
- Redness around the eye with eye pain or reduced movement
- Diabetic foot with a deep ulcer or no pulses
- Necrotising fasciitis
- Abscess needing drainage
- Septic arthritis under red skin
- DVT
- Diabetic foot osteomyelitis
- Orbital cellulitis
| Usual course | 5–7 days, longer only if slow to improve |
| Expect | Redness can spread slightly in the first 24–48 hours even on the right antibiotic |
| Review | At 48–72 hours: fever settling and less pain matter more than the edge |
| Admit | Eron class III–IV, septic, around the eye, rapidly spreading, can't take oral antibiotics |
| Two red legs | Rarely cellulitis: think venous eczema, lipodermatosclerosis, heart failure |
Check every dose against a current drug reference and your hospital protocol.
On this page
Cellulitis or something else?
| Condition | Clues |
|---|---|
| Cellulitis | One side, hot, tender, spreading, poorly defined edge, may have fever |
| Erysipelas | Raised, sharply defined, bright red edge; often face or leg; same treatment |
| Venous eczema or lipodermatosclerosis | Both legs, itchy or scaly, little pain, no fever |
| DVT | Calf swelling, less redness; Doppler if unsure |
| Gout or septic arthritis | Centred on a joint, painful movement |
| Abscess | Fluctuant, pointing: needs drainage |
| Necrotising fasciitis | Severe pain, systemic illness, rapid spread, skin changes late |
How sick are they?
- Oral antibiotic at home
- Review in 48 hours
- Oral may be enough
- Consider IV if not improving
- Admit
- IV antibiotics
- Senior review
- Resuscitate
- Surgery now for debridement
Pain out of proportion, rapid spread over hours, tense swelling beyond the red area, numbness, blisters, dusky skin, crackling. Antibiotics alone will not save the limb. Call surgery now.
Antibiotics
- Near the eyes or nose: co-amoxiclav is usually used; see an ophthalmologist if eye movement hurts or vision changes.
- Animal or human bite: co-amoxiclav, and think of rabies and tetanus.
- Suspected MRSA (previous MRSA, failed beta-lactam, pus): follow local policy; vancomycin IV if severe.
Don't forget
- Elevate the leg above the hip.
- Treat athlete's foot between the toes (terbinafine or clotrimazole cream).
- Pain relief: paracetamol.
- Diabetes: check sugar, inspect the foot fully, feel the pulses.
- Tetanus status if there is a wound.
Frequently asked questions
What is the best antibiotic for cellulitis?
For most adults, oral flucloxacillin for 5–7 days. With penicillin allergy, clarithromycin, doxycycline or clindamycin. IV treatment is for severe infection.
What is the difference between cellulitis and erysipelas?
Erysipelas is a superficial form with a raised, sharply defined, bright red edge. Cellulitis is deeper with a poorly defined edge. Treatment is the same.
How long does cellulitis take to improve on antibiotics?
Fever and pain usually improve within 48–72 hours. Redness can take longer and may spread slightly in the first day.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A patient has both shins red, scaly and itchy for months, with no fever. What is the likely diagnosis?
Answer: Venous eczema. Bilateral red legs are rarely cellulitis. Treat with emollients, steroid cream and compression if pulses are normal.
- Erysipelas: Usually one side, acute.
- Necrotising fasciitis: No.
- Bilateral cellulitis: Rare.
- DVT: Usually one leg.
2. A patient has leg redness, severe pain, BP 88/50 and purple blisters developing over 12 hours. What is the next step?
Answer: Emergency surgical debridement. Necrotising fasciitis is a surgical emergency.
- Elevation and observe: Fatal delay.
- Antihistamines: No.
- Oral flucloxacillin and review: Not for this.
- Ultrasound for DVT: Not first.
3. Which antibiotic is first choice for uncomplicated lower-leg cellulitis in a non-allergic adult?
Answer: Oral flucloxacillin. Flucloxacillin covers streptococci and staphylococci, the usual causes. Clarithromycin, doxycycline or clindamycin if penicillin-allergic.
- Oral nitrofurantoin: Urinary only.
- Oral ciprofloxacin: Poor streptococcal cover.
- Oral metronidazole: No streptococcal cover.
- IV meropenem: Far too broad.
4. A diabetic man has a red swollen thigh with pain far out of proportion to the signs, crepitus and rapidly spreading dusky skin. BP is falling. What is the key treatment?
Answer: Surgical debridement now. Necrotising fasciitis spreads along fascial planes: type 1 polymicrobial (diabetics, perineum — Fournier gangrene), type 2 group A streptococcus ± S. aureus. Pain out of proportion, rapid spread, crepitus, bullae, skin necrosis and systemic toxicity. LRINEC score helps but clinical suspicion leads to urgent surgical exploration and debridement plus IV broad-spectrum antibiotics (including clindamycin to inhibit toxin).
- Oral flucloxacillin: Oral antibiotics are inadequate for necrotising fasciitis.
- Elevation and review tomorrow: Waiting until tomorrow is fatal; necrotising fasciitis needs surgery now.
- MRI thigh before any surgery: Imaging delays surgery.
5. A 4-year-old has honey-coloured crusted lesions around the nose and mouth spreading to other children at nursery. He is well. What is the treatment?
Answer: Topical fusidic acid. Impetigo (S. aureus, Streptococcus pyogenes): non-bullous honey-crusted lesions or bullous (staphylococcal exfoliative toxin). Localised: topical antibiotic (fusidic acid, mupirocin) or hydrogen peroxide cream; extensive/bullous/systemically unwell: oral flucloxacillin (or erythromycin/cefalexin). Hygiene, keep off school until lesions crusted or 48 h after antibiotics. Post-streptococcal GN can follow.
- Oral prednisolone: Steroids worsen infection.
- Antifungal cream: Impetigo is not fungal.
- No treatment needed: Impetigo is contagious.
6. A 58-year-old diabetic man has his third episode of streptococcal cellulitis of the same leg in a year. Between attacks the leg is normal apart from mild oedema. Which of the following should be looked for and treated to prevent further attacks?
Answer: Fungal infection between the toes. Tinea pedis (athlete's foot) causes maceration and fissuring of the toe-web skin. These cracks let group A and other β-haemolytic streptococci enter the skin, so it is the commonest portal of entry for recurrent leg cellulitis. Each attack also damages lymphatics, adding to oedema and further risk. Treating the fungal infection with a topical azole or terbinafine, keeping toe webs dry and controlling oedema reduce recurrences; prophylactic penicillin is used if attacks continue.
- Hidden deep vein thrombosis: DVT does not cause recurrent streptococcal cellulitis.
- Occult osteomyelitis of the tibia: Osteomyelitis would not settle completely between attacks.
- Nasal carriage of MRSA: MRSA carriage does not explain recurrent streptococcal infection.
- Varicose veins of the thigh: Varicose veins do not create a portal of entry in the skin.
7. In a district hospital, a 58-year-old man with an anterior STEMI is given streptokinase because primary PCI is not available. Twenty minutes into the infusion his blood pressure falls from 130/80 to 85/50 mmHg. He has no wheeze, rash or bleeding, and the ECG shows sinus rhythm at 90/min. The most likely cause of the fall in pressure is:
Answer: Plasmin-generated bradykinin dilating vessels. Hypotension is common during streptokinase infusion and is related to the infusion rate. Plasmin generated in the blood activates the kallikrein system, releasing bradykinin, which dilates vessels. It usually responds to slowing or pausing the infusion, raising the legs and giving fluid. True anaphylaxis is uncommon and would bring wheeze or rash, while the Bezold–Jarisch reflex causes bradycardia, usually with inferior infarcts.
- IgE-mediated anaphylaxis to streptokinase: No wheeze, rash or angioedema.
- Bezold–Jarisch reflex from reperfusion: That gives bradycardia, mainly in inferior MI.
- Free-wall rupture with tamponade: Rupture is a later complication, typically days 3–5.
- Neutralising antistreptococcal antibodies: Antibodies cause treatment failure, not acute hypotension.
References and dose sources
- NICE NG141: Cellulitis and erysipelas, antimicrobial prescribing
- IDSA guideline: skin and soft tissue infections (Clin Infect Dis 2014)
- Flucloxacillin dosing: Drugs.com: Flucloxacillin (international) (see also our flucloxacillin dose page)
- Clarithromycin dosing: Drugs.com: Clarithromycin dosage (see also our clarithromycin dose page)
- Doxycycline dosing: Drugs.com: Doxycycline dosage (see also our doxycycline dose page)
- Clindamycin dosing: Drugs.com: Clindamycin dosage (see also our clindamycin dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.