A 62-year-old man presents with acute monoarthritis of the first MTP joint. Joint fluid shows negatively birefringent needle-shaped crystals under polarised light microscopy. Serum uric acid is 5.8 mg/dL (normal). The CORRECT interpretation is:

• A This rules out gout since serum uric acid is normal
• B This represents pseudogout; positively birefringent crystals confirm calcium pyrophosphate
• C Serum uric acid should be rechecked in 2 weeks before starting urate-lowering therapy
• D Gout cannot be excluded based on serum uric acid during an acute attack; crystal finding is diagnostic ✓

Explanation

During an acute gout attack, serum uric acid is frequently normal or even low due to uricosuric effect of acute-phase proteins and ACTH release. Serum uric acid should therefore not be used to exclude gout during an acute episode. Demonstration of negatively birefringent (yellow when parallel, blue when perpendicular to the compensator axis), needle-shaped monosodium urate crystals under polarised microscopy is diagnostic of gout regardless of serum levels.

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

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Written and medically reviewed by the StethoPrep medical team.