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Septic arthritis — SCE Rheumatology MCQ

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ModerateCrystal arthropathiesSeptic arthritisSCE Rheumatology

A 24-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: A — 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. Calcium, phosphate and magnesium is a plausible distractor, but it does not account for the key discriminator in the stem. Serum urate after the flare and Renal function would be more appropriate in a different clinical pattern or at another point in the pathway, while Plain radiograph is suboptimal for this presentation. Clinical pearl: autoantibodies are supportive tests and should be interpreted alongside phenotype, inflammatory markers, urinalysis and imaging.

Reference: NICE NG219; BSR hot swollen joint guideline; EULAR crystal arthropathy recommendations