Septic Arthritis / DVT / Tumour — the hot, swollen, or unexplained limb

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Who: Anyone. There is no demographic that protects them, though the risk sets differ for each of the three.
What: Infection in a joint · a clot in a deep vein · a primary or secondary bone or soft-tissue tumour.
How it behaves: Too hot, too swollen, too painful for what happened to it — or too persistent at night, in a limb nothing mechanical explains.

The one thing to take from this page:

You are not deciding which of the three it is. You are deciding whether this behaves like load-sensitive musculoskeletal pain — and if it doesn't, it gets seen today.

All three doors open onto the same corridor: same-day medical assessment. You do not have to know which one it was before you refer.

⚠️ No Australian guideline governs the septic-joint decision. The DVT limb has the THANZ guideline [22], and the bone-tumour limb has the national optimal care pathway for sarcoma (Cancer Australia), which lists the warning symptoms and sends suspected bone sarcoma for an X-ray and to a specialist sarcoma team. Anchored to the Scandinavian SANJO guideline for septic arthritis [1], the American Society of Hematology VTE diagnosis guideline [2] with CHEST for management [3], and the GEIS soft-tissue sarcoma guideline [4] alongside the UK bone sarcoma guidelines (Gerrand et al. 2025, not sighted). All are written for the team that receives your referral, not for you making it.

Five stages, in your course's order — with the special/screening questions doing most of the work, as they do on every red flag pattern.

MAIN PROBLEM

"My knee blew up overnight and I can't move it at all."
"My calf's been aching and this leg looks bigger than the other one."
"It's the night that's the problem. It wakes me and it doesn't matter what I do."

BODY CHART

Chart interpretation: This is a screening framework: labels mean possible septic arthritis, possible DVT and possible tumour. Appearance alone does not establish any of these diagnoses.

• Outline the whole limb, not the painful joint. Calf swelling that stops at the knee looks different from a knee effusion, and you will only see that if you drew both.
• Mark erythema and warmth as areas, not points.
• If there is a mass, draw it, measure it, and date the drawing.

BEHAVIOUR

This is where a septic joint separates itself. Ask what happens on the smallest possible movement.

HISTORY

• Sudden, atraumatic onset, or a trivial mechanism with a wildly disproportionate response.
• Recent injection, aspiration, surgery or procedure to that joint.
• A skin breach — cellulitis, an ulcer, a wound, a bite.
• Immobilisation, long travel, recent surgery, pregnancy or postpartum, hormone or oral contraceptive use, previous VTE, active malignancy.
• Previous cancer of any kind, at any time. Ask it plainly. Patients do not volunteer it.

SPECIAL / SCREENING QUESTIONS

These are the pattern, not the last box.

• New breathlessness or chest pain. In someone with a swollen calf or leg, that is a possible pulmonary embolism: ambulance, not a referral letter.
• Fever, sweats, rigors, feeling generally unwell. Ask about all four — patients under-report the vague ones.
• Unexplained weight loss. How much, over how long.
• Immunosuppression — medication, treatment, or a condition their doctor manages.
• Night pain that is not positional.

Three dangerous patterns, and the one feature that opens each door. Rank them — but note that the ranking changes nothing about where the patient goes.

The benign patterns that look similar — and why they don't clear it

• Acute gout or crystal arthropathy. Also hot, red, swollen and agonising. It is a genuine differential, and it is a medical diagnosis made on aspiration, not by you. A history of gout does not clear a septic joint; the two coexist.
• A large post-traumatic effusion. Has a mechanism, and the joint moves further before pain stops it.
• Calf strain. Has a mechanism, a moment, and localised tenderness — not a whole-limb size difference.

None of these three clears anything. The ranking tells you what to say in the referral. It does not tell you to keep the patient.