About This Page
This is a clinician-written, evidence-based guide aligned to the MCC Examination Objectives. It is structured by clinical presentation — the way the MCCQE tests and the way patients actually present. Management reflects current Canadian guidelines (CMA, CFPC, CPS). Always cross-reference with institutional protocols and clinical judgment.
The Bottom Line
- Classify SSTI first: non-purulent cellulitis, purulent abscess, bite/water exposure, diabetic foot, surgical-site or necrotizing infection
- Necrotizing fasciitis is a clinical/surgical emergency — pain out of proportion and rapid progression are key
- Abscess management is source control: incision and drainage is the key treatment
- Non-purulent cellulitis usually targets streptococci; purulent disease raises Staphylococcus aureus/MRSA considerations
- Reassess marked borders and toxicity; worsening pain or systemic illness should trigger escalation
Approach to the Presentation
Skin and soft tissue infection is common and can be over-treated or under-recognized. Start with anatomy and severity: epidermis/dermis, subcutaneous tissue, fascia, muscle, joint or bone. Then determine purulence, exposures and host risk. Non-purulent cellulitis is usually streptococcal; abscesses commonly involve Staphylococcus aureus and require drainage. Necrotizing fasciitis is rare but catastrophic and should be suspected from pain out of proportion, rapid progression and systemic toxicity.
Differential Diagnosis
| diagnosis | likelihood | key features | distinguishing test |
|---|---|---|---|
| Necrotizing fasciitis | must-not-miss | Severe pain out of proportion, rapid spread, toxicity, bullae, ecchymosis, anaesthesia, crepitus or shock | Clinical diagnosis requiring urgent surgical exploration; imaging must not delay surgery |
| Septic arthritis / osteomyelitis | must-not-miss | Overlying cellulitis with severe joint pain, inability to bear weight, reduced range of motion, diabetic foot ulcer or puncture wound | Joint aspiration for septic arthritis; MRI/bone imaging for osteomyelitis |
| Orbital cellulitis | must-not-miss | Eyelid swelling with fever, proptosis, painful/restricted eye movements, diplopia or decreased vision | Urgent CT orbit/sinuses and ophthalmology/ENT assessment |
| Non-purulent cellulitis / erysipelas | common | Warmth, erythema, tenderness and oedema; erysipelas has raised sharply demarcated borders | Clinical diagnosis; cultures usually low-yield unless severe or unusual |
| Cutaneous abscess / furuncle | common | Fluctuant tender collection, purulent drainage and surrounding cellulitis | Clinical; ultrasound if unclear; culture pus if recurrent, severe or treatment failure |
| Bite-associated infection | common | Dog/cat/human bite, puncture wounds or hand involvement; Pasteurella, anaerobes and oral flora | Clinical; X-ray for foreign body/fracture; culture infected drainage |
| Diabetic foot infection | common | Ulcer with erythema, warmth, purulence, malodour, neuropathy or vascular disease | Probe-to-bone, X-ray/MRI and deep tissue culture after debridement |
| Contact dermatitis / inflammatory mimic | common | Pruritus more than pain, bilateral/symmetric, exposure history, vesicles/scaling, no fever | Clinical pattern and lack of systemic features |
| DVT mimic | less common | Unilateral calf swelling and pain with VTE risk; erythema may mimic cellulitis | Compression ultrasound if DVT probability warrants |
| Herpes zoster | less common | Dermatomal burning pain with vesicular rash; early pain may precede rash | Clinical; VZV PCR if uncertain or immunocompromised |
Red Flags & Key History
Symptoms
Pain out of proportion, rapidly progressive erythema, bullae, necrosis, anaesthesia, crepitus or shock
Periorbital infection with proptosis, painful eye movements, diplopia or visual impairment
Severe joint pain, inability to move joint or bear weight
Diabetes, peripheral vascular disease, immunocompromise, injection drug use, bite, water exposure or recent surgery
Purulence, fluctuation or drainage — abscess/source control pathway
Bilateral leg erythema with chronic oedema suggests venous stasis dermatitis rather than cellulitis
Signs
Systemic toxicity: fever, tachycardia, hypotension or confusion
Crepitus, bullae, ecchymosis, dusky skin, necrosis or loss of sensation
Fluctuant abscess needing drainage
Lymphangitic streaking or regional lymphadenitis
Ulcer probing to bone, exposed tendon/bone or severe ischaemia in diabetic foot
Approach to Investigation
First-line
Clinical classification and marked bordersDocument size, location, purulence, lymphangitis, range of motion, neurovascular status and portal
CBC, CRP, creatinine if systemic illness or admission consideredNot required for every mild cellulitis; useful for severity and dosing
Ultrasound for suspected abscessUseful when fluctuance is unclear or cellulitis overlies deeper collection
Culture purulent drainage or abscess materialEspecially recurrent, severe, immunocompromised, community MRSA concern or treatment failure
Blood culturesFor severe illness, immunocompromise, unusual exposures or suspected bacteremia; low yield in mild cellulitis
Second-line
X-rayIf foreign body, gas, fracture, diabetic foot, puncture wound or osteomyelitis screen
CT/MRIFor deep infection, necrotizing infection when unclear, osteomyelitis, orbital cellulitis or abscess mapping
Joint aspirationMandatory when septic arthritis is suspected
Specialist
Urgent surgical consultationNecrotizing fasciitis, deep abscess, compartment concern, diabetic foot necrosis, hand infection or failed source control
Ophthalmology/ENTOrbital cellulitis or sinus-related complications
Infectious diseases / wound careRecurrent MRSA, immunocompromise, diabetic foot osteomyelitis or resistant organisms
Management Principles
AMMI Canada antimicrobial stewardship resources + Canadian Antibiotic Treatment Guidance1
Mild non-purulent cellulitis
- Treat streptococcal cellulitis with an appropriate oral beta-lactam when stable and able to absorb
- Elevate limb, manage portal of entry such as tinea pedis, mark borders and reassess
- Avoid routine blood cultures or imaging in uncomplicated mild cellulitis
2
Purulent abscess
- Incision and drainage is the key intervention
- Send culture when recurrent, severe, high-risk or treatment failure
- Add antibiotics for systemic illness, extensive cellulitis, immunocompromise, difficult sites or MRSA risk per local guidance
3
Severe or necrotizing infection
- ABCs, IV access, sepsis management and urgent surgical consultation
- Start broad empiric therapy covering streptococci, Staphylococcus aureus including MRSA if risk, Gram-negatives and anaerobes as indicated
- Clindamycin is often used for toxin suppression in suspected group A streptococcal necrotizing infection
4
Stewardship and follow-up
- Switch IV to oral when improving and able to absorb
- Use shortest effective duration based on response
- Reassess if pain, fever or erythema progresses after 24-48 hours
Complications & Pitfalls
- Missing necrotizing fasciitis: Pain out of proportion is the early clue; do not wait for crepitus.
- Antibiotics without drainage: Abscesses need source control.
- Overdiagnosing cellulitis: Bilateral red legs are more often venous stasis, oedema or dermatitis.
- Ignoring joints: A hot immobile joint is septic arthritis until proven otherwise.
- Excess IV therapy: Oral therapy is often appropriate when absorption is reliable.
MCCQE1 Exam Tips
- 1Nec fasc clue: severe pain out of proportion + rapid progression/systemic toxicity. Next best step is urgent surgery plus broad IV antibiotics
- 2Abscess = incision and drainage; antibiotics alone are insufficient for a fluctuant abscess
- 3Non-purulent cellulitis usually targets streptococci; purulent disease makes Staph aureus/MRSA relevant
- 4Orbital cellulitis red flags are proptosis, ophthalmoplegia, pain with eye movement and reduced vision
- 5Bilateral leg erythema should make you pause before diagnosing cellulitis
- 6Diabetic foot infection requires depth assessment, vascular status and osteomyelitis consideration
practicetest your knowledge on skin & soft tissue infection (cellulitis, abscess, necrotizing fasciitis)Apply what you've learnt with MCCQE1-style questions from the iatroX Q-Bank — infectious disease and beyond.
open q-bank