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Septic Arthritis — SCE Rheumatology MCQ

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EasyRheumatology EmergenciesSeptic ArthritisSCE Rheumatology

A 38-year-old woman presents with an acutely painful, hot, swollen knee. She is afebrile. Arthrocentesis yields 15 mL of turbid synovial fluid. Which set of analyses should be requested urgently on the aspirate?

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Correct answer: A — Gram stain, bacterial culture and polarised microscopy for crystals

The correct answer is A. An acute atraumatic hot, swollen joint must be investigated urgently for septic arthritis, even when the patient is afebrile. Synovial fluid should be sent for Gram stain and bacterial culture, while polarised microscopy identifies monosodium urate or calcium pyrophosphate crystals. Finding crystals does not by itself exclude concurrent infection, so microbiological testing remains necessary. Synovial glucose, protein, lactate dehydrogenase, pH and viscosity have insufficient discriminatory value to replace culture and microscopy. Cytology or flow cytometry may be appropriate for selected persistent or unexplained effusions, particularly when malignancy is suspected, but they are not the priority in acute monoarthritis.

Reference: The BMJ. Managing gout flares, Box 1: Distinguishing septic arthritis from a gout flare. BMJ 2026;392:r2248. https://www.bmj.com/content/392/bmj.r2248.full.pdf