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Pseudogout vs Septic Arthritis — SCE Infectious Diseases MCQ

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EasyBone & Joint InfectionPseudogout vs Septic ArthritisSCE Infectious Diseases

A 42-year-old man presents with a swollen painful right knee. Synovial fluid analysis shows 65,000 WCC/µL (95% neutrophils) with positively birefringent rhomboid-shaped crystals. Gram stain is negative. What is the diagnosis?

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Correct answer: C — Pseudogout (calcium pyrophosphate deposition disease — CPPD) — positively birefringent rhomboid crystals; NOT septic arthritis despite the high WCC

Positively birefringent rhomboid-shaped crystals under compensated polarised light microscopy are diagnostic of calcium pyrophosphate deposition disease (pseudogout/CPPD). This contrasts with gout which shows negatively birefringent needle-shaped monosodium urate crystals. Pseudogout can cause very high synovial WCC counts (sometimes >100,000) mimicking septic arthritis — the crystal identification is the distinguishing feature. Importantly, crystals and infection can coexist — if Gram stain is positive or clinical suspicion is high, treat for both. This patient's negative Gram stain supports pseudogout alone.

Reference: BSR 2017 – Gout/CPPD; NICE NG157 2024