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Presumed Septic Arthritis — RACP Adult Medicine MCQ

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ModerateRheumatologyPresumed Septic ArthritisRACP Adult Medicine

A 40-year-old man presents with acute monoarthritis of the right knee. Joint aspiration yields 45 mL of turbid fluid. Gram stain is negative. Crystal microscopy is negative. WCC is 65,000 cells/µL (95% neutrophils). What is the single most important initial management?

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Correct answer: B — Colchicine trial

With a high WCC (>50,000, predominantly neutrophils), negative crystals, and negative Gram stain, septic arthritis must be assumed until cultures return. Empirical IV antibiotics (flucloxacillin for suspected S. aureus; vancomycin if MRSA risk) should be commenced immediately. Joint drainage (arthroscopic washout or serial aspiration) is also essential. Gram stain sensitivity for septic arthritis is only ~50%, so a negative Gram stain does NOT exclude infection. Do not delay treatment for culture results.

Reference: eTG – 2025 – Antibiotic Guidelines; BSR – 2006 – Septic Arthritis Guidelines