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Septic arthritis vs transient synovitis: how the DipMSK actually tests it
Victoria3 min read
Written August 2026
Septic arthritis sits in the DipMSK syllabus as a condition that can imitate or complicate common joint presentations — and in children, the classic test is telling it apart from transient synovitis. That’s not an accident. Get it right in real life and a child goes home with paracetamol and a safety net. Get it wrong and a septic joint destroys cartilage while everyone waits and reviews in a week. The exam is testing whether you know which details matter enough to change that decision.
The two patterns, side by side
| Transient synovitis | Septic arthritis | |
|---|---|---|
| Child looks | Well in themselves | Systemically unwell |
| Fever | Absent or low-grade | High, often >38.5°C |
| Weight-bearing | Reduced but retained | Complete refusal |
| Hip position | Normal or mildly guarded | Held flexed, abducted, externally rotated |
| Passive movement | Tolerated, some discomfort | Resisted, acutely painful |
| Preceding illness | Recent viral URTI common | Not required |
| Action | Rest, analgesia, safety-net, review | Same-day assessment, aspirate before antibiotics |
Individually, several of these features overlap — a viral illness can precede either, and a nervous three-year-old can look reluctant to weight-bear whatever the underlying cause. The exam rarely relies on one feature in isolation; it builds a stem around a cluster of them, and the skill it’s testing is whether you weigh the cluster correctly rather than anchoring on the first data point you read.
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Why the Kocher criteria come up so often
The Kocher criteria — non-weight-bearing, fever above 38.5°C, ESR above 40, and white cell count above 12 — are widely taught as a way of stratifying the probability of septic arthritis in a limping child. They’re a useful mental checklist while you’re learning the pattern, but the DipMSK typically presents you with the clinical picture rather than a full set of blood results, so the exam is testing clinical pattern recognition more than criteria-scoring arithmetic. Know the criteria as a framework for how the risk builds; don’t expect every SBA to hand you all four numbers.
The trap questions to watch for
A few specific setups come up repeatedly, and each one is designed to catch a particular kind of shortcut:
The child who can still weight-bear “a bit.” This is often used to lure candidates toward transient synovitis when other features — high fever, a held and guarded hip, systemic upset — should override it. Partial weight-bearing lowers the probability of septic arthritis; it doesn’t rule it out.
The afebrile septic joint. Less common, but tested precisely because it breaks the “no fever = safe” heuristic. A hot, acutely painful, guarded joint with a systemically unwell child can still be septic arthritis even with a normal or borderline temperature, particularly in infants or a partially treated presentation.
The recurring limp. Stems describing a limp going back weeks, or worse in the evening after activity, are steering you away from an acute infective process and toward Perthes disease, a slipped upper femoral epiphysis, or a mechanical cause — the tempo of onset is doing a lot of diagnostic work, and it’s easy to skim past it while focused on the fever and the exam findings.
The one rule worth over-learning
If you take one thing from this topic into the exam, make it this: a febrile child who won’t weight-bear at all, with a held and guarded hip, is septic arthritis until proven otherwise — full stop, same-day assessment, aspirate before antibiotics. The exam isn’t looking for hedging in that scenario. It’s looking for candidates who recognise a surgical emergency and say so.
How this fits the platform
Our new limping child scenarios in the DipMSK question bank run through exactly this differential — transient synovitis, septic arthritis, Perthes, SUFE, growing pains and red-flag malignancy — with the reasoning spelled out for each wrong answer, not just the right one.
