A 62-year-old man presents with a swollen, hot right first metatarsophalangeal joint. Synovial fluid shows negatively birefringent needle-shaped crystals under polarised light microscopy. Serum uric acid is 5.8 mg/dL (normal). Which statement is most accurate about this scenario?

• A Normal serum uric acid excludes gout; another crystal arthropathy must be considered
• B Serum uric acid can be falsely normal or low during an acute gout attack due to uricosuric effect of IL-6 ✓
• C Pseudogout — positively birefringent crystals confirm calcium pyrophosphate
• D The low uric acid indicates allopurinol should not be started ever in this patient

Explanation

Serum uric acid is often low during an acute gout flare because interleukin-6 (and the acute-phase response) increases renal uric acid excretion transiently. Therefore, a normal serum uric acid during an acute attack does not exclude gout — synovial fluid crystal analysis (negatively birefringent, needle-shaped = monosodium urate) remains the gold standard. Calcium pyrophosphate crystals (CPPD/pseudogout) are rhomboid-shaped and positively or weakly positively birefringent. Urate-lowering therapy is indicated even if baseline SUA appears normal on flare-day testing if repeated measurements confirm hyperuricemia.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.