How do I know if this is cellulitis or something more dangerous?

The most important look-alike to exclude is necrotising fasciitis — a deep tissue infection with mortality around 25–30%. Warning signs requiring immediate emergency attendance are: pain severely out of proportion to the redness you can see, skin turning black or blistering, a crackling sensation under the skin, very rapid spread, and feeling genuinely unwell with fever or confusion. Bilateral redness on both legs without obvious fever usually indicates venous stasis dermatitis — antibiotic treatment is not appropriate and delays the correct diagnosis and care.

How long do I need to take antibiotics and what if they're not working?

Five days of oral flucloxacillin is as effective as ten days for most mild-to-moderate cellulitis, as shown in a randomised trial comparing the two durations. If after 48–72 hours the redness has not stopped spreading beyond the pen mark, or you develop fever or systemic symptoms, contact your GP before completing the course. Non-response may mean a missed abscess requiring drainage, an organism not covered by flucloxacillin such as MRSA, an incorrect diagnosis, or progression to a deeper infection needing inpatient treatment.

Why does cellulitis keep coming back?

Recurrent cellulitis almost always has an identifiable cause. Tinea pedis — athlete's foot — is the single most common entry point for lower limb cellulitis, responsible for more than half of recurrences. Treating the tinea effectively breaks the cycle in many cases. Other major contributors include lymphoedema, chronic venous insufficiency, obesity, atopic eczema or leg ulcers breaching the skin barrier, and poorly controlled diabetes. People with three or more episodes per year may benefit from prophylactic low-dose penicillin twice daily for six to twelve months, as shown in the PATCH II trial.

Do I need to go to hospital or see a specialist?

Most mild-to-moderate cellulitis is managed in general practice with oral antibiotics and a 48–72 hour review. Go to emergency directly for: high fever with rigors, vomiting, rapidly spreading redness despite antibiotics, extreme pain disproportionate to findings, skin blistering or blackening, any eye changes or proptosis suggesting orbital cellulitis, or feeling confused or very unwell. People with diabetes, facial involvement, or significant immune suppression have a lower threshold for hospital admission. Stable patients needing intravenous antibiotics can often be managed through Hospital in the Home programs.

What does 'mark the border' mean and why does it matter?

When cellulitis is first assessed, a skin pen or permanent marker is used to trace the outer edge of the redness. At the review appointment 48–72 hours later, this mark shows objectively whether the infection is responding — redness contained within the mark — or spreading further, which requires management reassessment and possible hospital referral. Taking a photograph of the marked border allows tracking of changes between reviews and provides useful documentation. This simple step changes the review from subjective reporting to objective comparison.

Cellulitis

Cellulitis: diagnosis, treatment, and prevention in Australian general practice

By Dr HB Lo, FACRRM Reviewed 14 June 2026 10 min read

Cellulitis is a bacterial infection of the dermis and subcutaneous fat, almost always unilateral, presenting as an expanding area of redness, warmth, swelling, and tenderness. Beta-haemolytic streptococci and Staphylococcus aureus are the main organisms.

Oral flucloxacillin for five days is first-line treatment in Australia for mild-to-moderate cellulitis without systemic features. Mark the edge of redness with a skin pen and review in 48–72 hours. Bilateral leg redness is almost always venous stasis dermatitis — not cellulitis — and antibiotics are unnecessary.

Cellulitis is one of the most common reasons for acute antibiotic prescribing in Australian general practice, and one of the most common causes of hospital admission for skin infection. The condition accounts for approximately 10–25 cases per 1,000 person-years in the community, and around 30% of people who have one episode will have another within three years.

Despite its frequency, cellulitis is frequently misdiagnosed. The most common error is treating bilateral leg redness as cellulitis when it is almost always chronic venous stasis dermatitis — a different condition entirely that does not respond to antibiotics. The second major risk is failing to recognise necrotising fasciitis, a deep tissue emergency, before it progresses.

Cellulitis disproportionately affects people with diabetes, obesity, lymphoedema, chronic venous insufficiency, and skin conditions that compromise the barrier — atopic eczema, tinea pedis, and leg ulcers. Aboriginal and Torres Strait Islander communities, particularly in remote areas, bear a higher burden of streptococcal skin infection and impetigo, which also contributes to rheumatic fever and rheumatic heart disease.

This article follows Therapeutic Guidelines (eTG) Antibiotic: Skin and soft tissue infections and AMH guidance.

A. Core clinical — the AU general practice framework

Definition and classification

Cellulitis — diffuse acute infection of the dermis and subcutaneous fat; typically S. pyogenes (group A streptococcus, GAS) or Staphylococcus aureus (MSSA in most community cases).

Erysipelas — superficial cellulitis with a sharply demarcated raised edge; predominantly S. pyogenes; classic presentation on the face (children) or lower limb.

Necrotising fasciitis — deep infection of the fascia and muscle with rapid necrosis; mortality 25–30%; surgical emergency.

Lymphangitis — red streaking along lymphatic channels; associated regional lymphadenopathy; almost always GAS.

Orbital (postseptal) cellulitis — posterior to the orbital septum; proptosis, pain on eye movement, ophthalmoplegia; emergency with CT orbits and IV antibiotics.

Periorbital (preseptal) cellulitis — anterior to the orbital septum; less serious; usually managed with oral antibiotics.

History

Key points:

Onset, speed of evolution, pain severity relative to appearance
Skin barrier breach — tinea pedis, eczema, ulcer, trauma, recent surgery, insect bite, IV drug use
Animal or human bite
Water exposure (saltwater, freshwater, shellfish handling)
Systemic symptoms — fever, rigors, vomiting
Comorbidities — diabetes, peripheral vascular disease, lymphoedema, venous insufficiency, obesity, immunosuppression
Anticoagulants or immunosuppressants
Penicillin allergy — clarify whether anaphylaxis (severe) or rash (non-severe)
Vaccination status — tetanus
Prior cellulitis episodes

Examination

Vital signs — temperature, heart rate, blood pressure, respiratory rate, SpO₂, conscious state (sepsis screen)
Local findings — erythema (mark the border), warmth, tenderness, swelling, fluctuance, blistering, skin colour
Red flags — pain disproportionate to findings, rapidly advancing border, bullae, necrosis, crepitus → suspect necrotising fasciitis
Entry site — inspect web spaces (tinea), ulcer margins, scratch, bite wound
Lymphangitis / lymphadenopathy
Bilateral vs unilateral — bilateral involvement without systemic features is almost never true cellulitis; look for chronic venous changes (haemosiderin staining, lipodermatosclerosis)
DVT differential — unilateral calf swelling without redness; assess need for Doppler ultrasound

Investigations

Most mild cellulitis: clinical diagnosis only; bloods not routinely required.

When indicated:

FBC, CRP, urea and electrolytes — moderate-severe or uncertain diagnosis
Blood cultures — only if systemic features or immunocompromised; yield under 5% in mild uncomplicated cellulitis
Wound swab — only if open wound, abscess, treatment failure, or atypical organism suspected
Soft tissue ultrasound — if abscess is suspected (fluctuance, failure to respond)
D-dimer and lower limb Doppler — if DVT is in the differential
CT or MRI — if necrotising fasciitis is suspected; do NOT delay surgical referral for imaging
HbA1c — all cases of recurrent or severe cellulitis (diabetes screen)
LRINEC score (CRP, white cell count, haemoglobin, sodium, creatinine, glucose) — adjunct for necrotising fasciitis risk stratification; not diagnostic alone

Differential diagnosis

Condition	Key discriminator
Cellulitis	Unilateral; expanding erythema + warmth + tenderness
Necrotising fasciitis	Disproportionate pain; rapid spread; bullae; crepitus; systemic — surgical emergency
Erysipelas	Sharply raised demarcated border; predominantly S. pyogenes
Venous stasis dermatitis	Bilateral; chronic; venous signs; no fever; no acute onset
Deep vein thrombosis	Unilateral calf swelling; less erythema; Doppler positive
Gout or pseudogout	Involves joint; crystals on aspirate
Contact dermatitis	Distribution matches contactant; itch-predominant
Herpes zoster	Dermatomal vesicular rash
Eosinophilic cellulitis (Wells syndrome)	Recurrent; peripheral eosinophilia; biopsy diagnostic

Treatment

Mild-to-moderate (no systemic features):

Per eTG Antibiotic:

Flucloxacillin 500 mg four times daily for 5 days — covers both streptococci and MSSA S. aureus; extend to 7–10 days if response is slow
Non-severe penicillin allergy (rash without anaphylaxis): cefalexin 500 mg four times daily OR clindamycin 450 mg three times daily × 5–7 days
Severe penicillin allergy (anaphylaxis): clindamycin 450 mg three times daily; IV vancomycin if admission required

Adjuncts:

Mark the erythema border with a skin pen; review at 48–72 hours
Elevate the affected limb; reduces swelling and speeds recovery
Paracetamol for analgesia; NSAIDs with caution (dehydration risk; weak signal for worsening necrotising infection)
Treat the entry site — tinea pedis with terbinafine or clotrimazole; eczema flare with appropriate topical therapy

Severe cellulitis (systemic features, rapid spread, or failed oral therapy):

Emergency Department or Hospital in the Home (HITH)
IV flucloxacillin 2 g six-hourly OR cefazolin 2 g eight-hourly
Broaden if MRSA suspected or necrotising fasciitis cannot be excluded

B. Evidence appraisal — key trials

Duration of treatment

Hepburn et al (Arch Intern Med 2004) randomised adults with uncomplicated lower limb cellulitis to 5-day versus 10-day antibiotic courses. Five-day treatment was non-inferior for clinical cure at 14 days. eTG and AMH both endorse 5-day courses for mild-to-moderate cellulitis, extending only when the clinical response is slow.

Prophylaxis for recurrent cellulitis

PATCH II (NEJM 2013, Thomas et al) randomised adults with two or more cellulitis episodes in the past three years to prophylactic phenoxymethylpenicillin 250 mg twice daily or placebo for 12 months. The penicillin arm had significantly fewer episodes (8% vs 37% with recurrence). Benefit persisted during the prophylaxis period. This provides the evidence base for offering prophylactic penicillin to patients with three or more episodes per year after reversible risk factors have been addressed.

Empirical MRSA cover

Community-acquired MRSA accounts for under 10% of skin infections in most Australian regions. Routine empirical MRSA cover is not recommended in the absence of specific risk factors: recent hospitalisation or residential aged care, prior MRSA colonisation or infection, IV drug use, or household contact with MRSA-positive individual. When MRSA is suspected, first-line oral agents are trimethoprim-sulfamethoxazole or doxycycline (check local susceptibility patterns).

NSAIDs in cellulitis

A small number of case series have raised concern that NSAIDs may mask the progression of necrotising fasciitis by suppressing early inflammatory signs. Evidence is observational and debated. Paracetamol is preferred for analgesia. NSAIDs are not contraindicated but should be used with caution, particularly in patients who are dehydrated.

Hospital in the Home

HITH for IV antibiotic delivery is safe and preferred by patients compared with inpatient admission in stable severe cellulitis. It is available through most Australian public hospital networks.

C. Special clinical scenarios

Necrotising fasciitis

A surgical emergency with mortality around 25–30%. Do not delay surgical referral while awaiting CT or MRI. Clinical features that demand immediate escalation:

Pain severely disproportionate to visible findings — the hallmark
Rapidly advancing border — spreading despite antibiotics
Bullae, skin necrosis, darkening, or blackening
Crepitus (gas in soft tissue)
Anaesthesia of the overlying skin (late sign — nerve necrosis)
Systemic toxicity (high fever, rigors, haemodynamic instability)

Empirical IV regimen (broaden immediately): piperacillin-tazobactam + vancomycin + clindamycin (the last for toxin suppression). Urgent surgical debridement is definitive.

Orbital versus periorbital cellulitis

Periorbital (preseptal) — swelling anterior to the orbital septum; no proptosis, no pain on eye movement, no ophthalmoplegia. Managed with oral antibiotics (usually cefalexin or amoxicillin-clavulanate) and close review.

Orbital (postseptal) — proptosis, pain on eye movement, ophthalmoplegia, or reduced visual acuity. Emergency: CT orbits (axial + coronal with contrast), IV antibiotics, ENT and/or ophthalmology referral immediately.

Bite wounds

Cat and dog bites — amoxicillin-clavulanate 875/125 mg twice daily for 5 days covers Pasteurella multocida, anaerobes, and S. aureus; Pasteurella causes rapid-onset (hours) intense cellulitis
Human bites — amoxicillin-clavulanate; high risk of Eikenella corrodens and anaerobes; hand bites need surgical review (joint and tendon sheath risk)
Tetanus — check vaccination status per Australian Immunisation Handbook for every wound
Australian bat lyssavirus or overseas rabies exposure — urgent public health notification; post-exposure prophylaxis
Saltwater wounds — add doxycycline 100 mg twice daily for Vibrio vulnificus (high mortality in cirrhosis or immunocompromise)
Freshwater wounds — add ciprofloxacin for Aeromonas hydrophila

Recurrent cellulitis management

When three or more episodes occur per year, systematically address reversible risk factors before considering prophylaxis:

Treat tinea pedis (entry site in >50% of lower limb cellulitis) — terbinafine cream or oral terbinafine for resistant tinea; see eTG dermatology
Lymphoedema management — compression bandaging; referral to lymphoedema physiotherapist
Skin care — daily emollient; prompt treatment of any skin breach
Weight management — obesity increases lymphoedema and venous insufficiency risk
Glycaemic optimisation if diabetes is present
Treat venous insufficiency — graduated compression stockings; vascular review for ulcers
MRSA decolonisation if recurrent MRSA — mupirocin nasal ointment plus daily chlorhexidine wash, per protocol

After addressing reversible factors, prophylactic phenoxymethylpenicillin 250 mg twice daily for 6–12 months is the evidence-based choice per PATCH II. For penicillin allergy, erythromycin or clindamycin are alternatives.

D. Australian operations

MBS billing

Items 23/36/44 — standard GP consultations (mild, moderate, complex)
Item 10997 — practice nurse wound review; dressing change
Items 707/715 — health assessments (75+ and Aboriginal and Torres Strait Islander patients)
Items 965/967 — GP Chronic Disease Management Plan for recurrent cellulitis with comorbidity (lymphoedema, diabetes, venous insufficiency, obesity); enables allied health referrals to lymphoedema physiotherapist, podiatrist, dietitian
Pathology items — FBC (65070), CRP (66509), UEC (66500), LFT (66512), blood culture (69319), wound swab (69300) via MBS Online
HITH — state-funded; not directly an MBS item but linked to hospital admission avoidance programs

PBS

Per PBS schedule:

Flucloxacillin, cefalexin, amoxicillin-clavulanate, doxycycline, trimethoprim-sulfamethoxazole — general schedule; no authority required
Clindamycin — Authority Required
Phenoxymethylpenicillin — general schedule (prophylaxis)
Vancomycin IV — Section 100 Highly Specialised Drugs; inpatient / HITH setting

Referral pathways

Emergency Department — necrotising fasciitis, orbital cellulitis, sepsis, severe periorbital, failed oral therapy
General surgery / plastic surgery — suspected necrotising fasciitis, deep abscess, bite wounds involving hand
ENT / ophthalmology — orbital cellulitis
Infectious diseases — atypical organism, MRSA, immunocompromise, unusual water-exposure organisms
Vascular surgery — severe limb-threatening diabetic foot infection, chronic venous insufficiency
Podiatry / diabetic foot service — high-risk foot

E. Special populations

Pregnancy: flucloxacillin and cefalexin are safe throughout pregnancy. Avoid trimethoprim-sulfamethoxazole in the first trimester and third trimester. Doxycycline is avoided throughout pregnancy. Severe cellulitis in pregnancy warrants Emergency Department assessment.

Children: cefalexin or flucloxacillin oral are first-line. Children have a lower threshold for hospital admission — particularly for periorbital or orbital cellulitis, large surface area involvement, or any immunocompromise. Periorbital cellulitis in a child requires CT orbits to exclude orbital extension.

Older adults: cellulitis may present atypically — minimal fever but delirium, falls, or anorexia. Sepsis signs may be blunted. Falls risk during IV treatment at home should be assessed for HITH suitability. Penicillin allergy history in older adults should be clarified — many labelled allergies are non-severe rashes that do not preclude cefalexin or amoxicillin-clavulanate.

Aboriginal and Torres Strait Islander people: higher burden of skin infection, particularly in remote communities. Streptococcal skin infections are an important pathway to acute rheumatic fever and rheumatic heart disease — early and complete antibiotic treatment matters beyond the immediate episode. The ATSI health assessment (item 715) provides an opportunity to screen for skin conditions and address underlying risk factors.

When to escalate

Necrotising fasciitis signs (disproportionate pain, bullae, crepitus, rapid spread) — Emergency Department immediately; surgical emergency
Orbital cellulitis (proptosis, pain on eye movement, ophthalmoplegia) — Emergency Department and urgent CT orbits
Sepsis — Emergency Department
Failure to respond at 48–72 hours — reassess diagnosis; consider abscess, MRSA, atypical organism, necrotising
Diabetic foot infection with systemic features or deep tissue involvement — multidisciplinary high-risk foot service
Bite wounds to the hand — surgical review (tendon and joint risk)
Immunocompromised patient with any severe or atypical infection — low threshold for inpatient care

What this article is and is not

This is general health information drawn from Australian clinical guidelines — Therapeutic Guidelines (eTG) Antibiotic, AMH, and ASID — and named clinical trials. It is not personal medical advice and does not create a doctor–patient relationship. Decisions about diagnosis, antibiotic choice, and hospital referral are made individually by your GP or treating clinician based on clinical assessment.

For consumer information: HealthDirect — Cellulitis, Better Health Channel — Cellulitis.

Sources cited

Frequently asked questions

How do I know if this is cellulitis or something more dangerous?

The most important look-alike to exclude is necrotising fasciitis — a deep tissue infection with mortality around 25–30%. Warning signs requiring immediate emergency attendance are: pain severely out of proportion to the redness you can see, skin turning black or blistering, a crackling sensation under the skin, very rapid spread, and feeling genuinely unwell with fever or confusion. Bilateral redness on both legs without obvious fever usually indicates venous stasis dermatitis — antibiotic treatment is not appropriate and delays the correct diagnosis and care.
How long do I need to take antibiotics and what if they're not working?

Five days of oral flucloxacillin is as effective as ten days for most mild-to-moderate cellulitis, as shown in a randomised trial comparing the two durations. If after 48–72 hours the redness has not stopped spreading beyond the pen mark, or you develop fever or systemic symptoms, contact your GP before completing the course. Non-response may mean a missed abscess requiring drainage, an organism not covered by flucloxacillin such as MRSA, an incorrect diagnosis, or progression to a deeper infection needing inpatient treatment.
Why does cellulitis keep coming back?

Recurrent cellulitis almost always has an identifiable cause. Tinea pedis — athlete's foot — is the single most common entry point for lower limb cellulitis, responsible for more than half of recurrences. Treating the tinea effectively breaks the cycle in many cases. Other major contributors include lymphoedema, chronic venous insufficiency, obesity, atopic eczema or leg ulcers breaching the skin barrier, and poorly controlled diabetes. People with three or more episodes per year may benefit from prophylactic low-dose penicillin twice daily for six to twelve months, as shown in the PATCH II trial.
Do I need to go to hospital or see a specialist?

Most mild-to-moderate cellulitis is managed in general practice with oral antibiotics and a 48–72 hour review. Go to emergency directly for: high fever with rigors, vomiting, rapidly spreading redness despite antibiotics, extreme pain disproportionate to findings, skin blistering or blackening, any eye changes or proptosis suggesting orbital cellulitis, or feeling confused or very unwell. People with diabetes, facial involvement, or significant immune suppression have a lower threshold for hospital admission. Stable patients needing intravenous antibiotics can often be managed through Hospital in the Home programs.
What does 'mark the border' mean and why does it matter?

When cellulitis is first assessed, a skin pen or permanent marker is used to trace the outer edge of the redness. At the review appointment 48–72 hours later, this mark shows objectively whether the infection is responding — redness contained within the mark — or spreading further, which requires management reassessment and possible hospital referral. Taking a photograph of the marked border allows tracking of changes between reviews and provides useful documentation. This simple step changes the review from subjective reporting to objective comparison.

Source quality

Sources grouped by evidence tier. AU primary tier first; international where AU is silent or lagging; named-author reconstruction where guidelines have not yet caught up. How tiers work.