Comparison of Polarized Light Microscopic Findings of Synovial Fluid from a Joint with Gout and a Joint with Pseudogout.
| Gout | Pseudogout | |
|---|---|---|
| Crystal | Urate | Calcium pyrophosphate dehydrate |
| Shape | Needle | Rhomboid or rectangular |
| Birefringence | Negative | Positive |
| Color of crystals parallel to axis of red-plate compensator | Yellow | Blue |
Pearl: For the crystal color, use the mnemonic ABC (alignment, blue, calcium). If the crystal aligned with the red-plate compensator is blue, it is CPPD. Urate crystals are the opposite, being yellow when parallel to the compensator. Crystals within WBCs are more indicative of a crystalline arthropathy than free-floating crystals. Note that hydroxyapatite crystals are too small to see with polarized light
