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Septic arthritis — SCE Rheumatology MCQ

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ModerateMusculoskeletal investigationsSeptic arthritisSCE Rheumatology

A 38-year-old woman has fever and a rapidly swollen painful knee. Examination shows there is severe restriction of passive movement. Investigations show synovial fluid has high neutrophils and Gram-positive cocci. The consultant wants the next test most likely to confirm the suspected process or guide treatment. The result will change diagnosis, staging or immediate treatment. What is the most appropriate investigation?

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Correct answer: D — Synovial fluid microscopy and culture

Synovial fluid microscopy and culture is the best answer because synovial fluid microscopy and culture is the investigation most likely to resolve the immediate diagnostic or monitoring question in septic arthritis. Ultrasound-guided aspiration is a plausible distractor, but it does not account for the key discriminator in the stem. Plain radiographs of hands and Urinalysis would be more appropriate in a different clinical pattern or at another point in the pathway, while Anti-CCP antibody is suboptimal for this presentation. Clinical pearl: autoantibodies are supportive tests and should be interpreted alongside phenotype, inflammatory markers, urinalysis and imaging.

Reference: BSR hot swollen joint guideline; NICE NG100; EULAR imaging recommendations