Which synovial fluid analysis findings confirm a diagnosis of gout or pseudogout?

Updated: Jan 26, 2021
  • Author: Bruce M Rothschild, MD; Chief Editor: Herbert S Diamond, MD  more...
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Answer

Urate crystals are shaped like needles or toothpicks with pointed ends (see the first image below). Under polarizing light microscopy, urate crystals are yellow when aligned parallel to the axis of the red compensator and blue when aligned across the direction of polarization (ie, they exhibit negative birefringence). Finding negatively birefringent urate crystals (see the second image below) firmly establishes the diagnosis of gouty arthritis.

Gout. Needles of urate crystals seen on polarizing Gout. Needles of urate crystals seen on polarizing microscopy.
Gout. Strongly negative birefringent, needle-shape Gout. Strongly negative birefringent, needle-shaped crystals diagnostic of gout obtained from acutely inflamed joint.

Pseudogout crystals (CPP) are rod-shaped with blunt ends and are positively birefringent. Thus, pseudogout crystals are blue when aligned parallel to the slow ray of the compensator and yellow when they are perpendicular.

Crystals must be distinguished from birefringent cartilaginous or other debris. Debris may have fuzzy borders and may be curved, whereas crystals have sharp borders and are straight. As alkalization reduces uric acid crystal solubility and the enzyme uricase can “dissolve” these crystals, reduction by addition of sodium hydroxide or uricase to suspected gout crystal can be helpful.

Corticosteroids injected into joints have a crystalline structure that can mimic either MSU or CPP crystals. They can be either positively or negatively birefringent.


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