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Irritable Hip — RACP Paediatrics MCQ

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ModerateEmergency PaediatricsIrritable HipRACP Paediatrics

A 4-year-old presents with a limp and refusal to weight-bear on the right leg. He has a temperature of 38.8°C. CRP is 45 mg/L. Hip ultrasound shows a moderate effusion. What is the most important investigation to differentiate septic arthritis from transient synovitis?

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Correct answer: B — Ultrasound-guided joint aspiration

Joint aspiration with synovial fluid analysis (cell count, Gram stain, culture) is the definitive investigation to differentiate septic arthritis from transient synovitis. Kocher criteria (fever >38.5°C, non-weight-bearing, ESR >40, WCC >12 × 10⁹/L) help stratify risk but do not replace aspiration. A WCC >50,000/mm³ in synovial fluid suggests septic arthritis.

Reference: RCH Melbourne – 2024 – Clinical Practice Guidelines: Limp