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Gram-Negative Septic Arthritis on Tocilizumab — SCE Rheumatology MCQ

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HardRheumatology EmergenciesGram-Negative Septic Arthritis on TocilizumabSCE Rheumatology

A 60-year-old woman with rheumatoid arthritis treated with tocilizumab presents with a 24-hour history of severe pain, swelling and restricted movement of the right wrist. She has recurrent urinary tract infections and reports dysuria for 2 days. Her temperature is 38.1°C and CRP is 5 mg/L. Wrist aspiration produces purulent fluid containing Gram-negative bacilli on microscopy. Which interpretation is most appropriate?

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Correct answer: B — Enterobacterales such as Escherichia coli are plausible; tocilizumab suppresses IL-6-driven CRP production, so the low CRP does not exclude septic arthritis.

The best answer is B. Purulent synovial fluid containing Gram-negative bacilli establishes a strong working diagnosis of Gram-negative septic arthritis. With concurrent urinary symptoms and recurrent UTIs, haematogenous infection by Enterobacterales, particularly E. coli, is plausible, although definitive species identification requires culture. Tocilizumab blocks IL-6 receptor signalling; because IL-6 drives hepatic acute-phase protein synthesis, severe infection may produce little or no CRP rise. A CRP of 5 mg/L therefore cannot reassure. Gram stain alone cannot establish Pseudomonas, and tocilizumab causes biological suppression rather than assay interference. Gonococci are Gram-negative diplococci, not bacilli. Staphylococcus aureus remains the commonest overall cause of septic arthritis but conflicts with the reported microscopy. A rheumatoid flare does not explain purulent fluid containing organisms.

Reference: RoActemra 20 mg/mL concentrate for solution for infusion, Summary of Product Characteristics, sections 4.4 and 5.1, updated 19 March 2026. https://www.medicines.org.uk/emc/product/6673/smpc