Simple non-staining method to demonstrate urate crystals in formalin-fixed, paraffin-embedded skin biopsies.

Background: Gouty tophi classically occur as nodules over joints and the helix. The ideal fixative for preservation of gout crystals has traditionally been alcohol because the crystals are formalin and water soluble. However, most biopsies are submitted in formalin fixative, which results in dissolution of urate crystals leaving behind a non-specific pale amorphous area. Although complex staining methods to show urate crystals in tissue have been described, the present study elucidates a simple non-staining method utilizing a thick unstained coverslipped microscopy slide that allows detection and confirmation of birefringence of urate crystals in formalin-fixed, paraffin-embedded tissue.

Methods: Twenty-nine cases of cutaneous gouty tophi were evaluated using a hematoxylin-eosin (H&E)-stained section, a 10-microm unstained coverslipped section and a 4-microm unstained coverslipped section. In all cases, the specimen was received and submitted in formalin.

Results: Polarizable crystals were not identified in any of the H&E sections. The use of the thicker unstained coverslipped section was more sensitive than the standard 4 microm section by recognizing the characteristic urate crystals in 48% and 38% of the cases, respectively.

Conclusions: This inexpensive adjunctive tool can be used to document gout crystals in almost half of skin biopsies.