Compensated polarized light microscopy. Identification of crystals in synovial fluids from gout and pseudogout

Abstract

In an attack of arthritis suspected to be due to gout or pseudogout, the diagnosis can be rapidly and accurately established by identifying monosodium urate or calcium pyro-phosphate crystals in the synovial fluid from an affected joint. Polarized light microscopy facilitates location of crystals, while the addition of a 1st-order red compensator to the system imparts characteristic color changes and allows definitive separation of urate from pyro-phosphate crystals. Urates show strong negative birefringence, and pyrophosphates show weak positive birefringence. A slide made from urates aspirated from a tophus provides a useful reference in learning to distinguish between these 2 crystal types. Although.conversion of the standard laboratory microscope is possible, a polarizing microscope with a rotating stage is recommended for crystal identification.