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CPPD Pseudogout — RACP Adult Medicine MCQ

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HardRheumatologyCPPD PseudogoutRACP Adult Medicine

A 73-year-old has an acutely hot knee, fever and synovial fluid containing positively birefringent calcium-pyrophosphate crystals. Gram stain is negative and synovial WCC is 58,000/microlitre. What is the safest interpretation?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: E — Culture synovial fluid and continue assessing for concurrent septic arthritis

Explanation lettering: D = shown as B · B = shown as C · E = shown as D · C = shown as E

C is correct because crystal arthritis and septic arthritis can coexist; Gram stain has limited sensitivity and cultures plus clinical assessment remain essential. D treats crystals as mutually exclusive with infection. E overstates a negative Gram stain. A ignores broad overlap in synovial cell counts. B is a chronic structural clue, not an infection-exclusion test. If the patient is systemically unwell, blood cultures, orthopaedic involvement and empiric antibiotics should not await final synovial culture.

Reference: South Western Sydney Local Health District, crystal arthropathy referral guidance: https://www.swslhd.health.nsw.gov.au/Liverpool/rheumatology%5Ccontent/pdf/2022_-_RHEUMATOLOGY_REFERRAL_GUIDELINES-.pdf