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Hot swollen joint — SCE Rheumatology MCQ

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ModerateMusculoskeletal investigationsHot swollen jointSCE Rheumatology

A 70-year-old man with diabetes presents with fever and an acutely hot, swollen knee. Examination shows a tense effusion and severe pain on both active and passive movement. His CRP is 210 mg/L. A knee radiograph shows osteoarthritis but no acute bony abnormality. What is the most appropriate next investigation?

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Correct answer: A — Urgent knee aspiration for microbiology and crystal analysis

The correct answer is A. Fever, diabetes, a tense monoarticular effusion, marked restriction of passive movement and CRP 210 mg/L make native-joint septic arthritis a diagnostic emergency. The knee should be aspirated urgently, with synovial fluid sent for Gram stain and culture and for crystal microscopy; crystal arthritis and infection can coexist. A negative Gram stain does not exclude infection. Radiographic osteoarthritis is common at this age and does not explain the systemic inflammatory presentation. Serum urate alone cannot diagnose the acute episode or exclude sepsis. Anti-CCP and HLA-B27 address other inflammatory arthropathies, while delayed MRI would inappropriately postpone the decisive investigation.

Reference: Ravn C et al. Guideline for management of septic arthritis in native joints (SANJO), sections on diagnostic approach and synovial fluid analysis, 2023. https://pubmed.ncbi.nlm.nih.gov/36756304/