How do the crystals of gout and pseudogout appear on microscopic analysis?

Updated: Jan 26, 2021
  • Author: Bruce M Rothschild, MD; Chief Editor: Herbert S Diamond, MD  more...
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In gout, crystals of monosodium urate (MSU) appear as needle-shaped intracellular and extracellular crystals. When examined with a polarizing filter and red compensator filter, they are yellow when aligned parallel to the slow axis of the red compensator but turn blue when aligned across the direction of polarization (ie, they exhibit negative birefringence). Negatively birefringent urate crystals are seen on polarizing examination in 85% of specimens.

Microscopic analysis in pseudogout shows calcium pyrophosphate (CPP) crystals, which appear shorter than MSU crystals and are often rhomboidal. Under a polarizing filter, CPP crystals change color depending upon their alignment relative to the direction of the red compensator. They are positively birefringent, appearing blue when aligned parallel with the slow axis of the compensator and yellow when perpendicular.

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