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Septic Arthritis Immunosuppressed — RACP Adult Medicine MCQ

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ModerateRheumatologySeptic Arthritis ImmunosuppressedRACP Adult Medicine

A 65-year-old woman with rheumatoid arthritis on long-term low-dose prednisolone (5 mg daily) develops an acutely swollen, painful knee. Joint aspiration reveals a WCC of 55,000 cells/µL with 90% neutrophils but no crystals and negative Gram stain. Culture is pending. While awaiting culture results, how should this be managed?

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Correct answer: B — Treat as septic arthritis with empirical IV antibiotics

In an immunosuppressed patient (on prednisolone) with an acutely inflamed joint and WCC >50,000 cells/µL with neutrophilic predominance, septic arthritis must be assumed until culture results return. Gram stain is only ~50% sensitive for septic arthritis – a negative Gram stain does NOT exclude infection. Empirical IV antibiotics (flucloxacillin for suspected S. aureus; vancomycin if MRSA risk) should be commenced immediately alongside repeated joint aspiration/washout. Delayed treatment of septic arthritis results in irreversible joint destruction.

Reference: eTG – 2025 – Antibiotic Guidelines