A tophus specimen should be sent dry to the pathology department, and not be put in formalin.[notes 2] A touch prep can be done initially, and at least if urate crystals are not initially detected, the specimen is submitted in 100% alcohol for microscopy evaluation.[notes 2]
On a touch prep, look for needle-shaped crystals of urate. On polarized light, these will have negative birefriengence.
Uric acid crystals in polarized light, showing negative birefringence, with yellow color when aligned parallel to the axis of the red compensator, and blue when aligned perpendicularly to it.
- Bruce M Rothschild. Gout and Pseudogout Workup. Medscape. Updated: Jun 30, 2020