The definitive diagnosis of gout and pseudogout often rests upon the microscopic demonstration of the specific crystal. However, other structures may be found in the synovial fluid which may be confused with sodium urate or calcium pyrophosphate; these include calcium oxalate crystals, collagen fibrils or cartilage fragments, cholesterol crystals, metallic fragments after prosthetic arthroplasty, and microcrystalline corticosteroid esters. Artifacts such as scratches in slides or cover slips, and dirt particles may require differentiation. Proper interpretation of crystals found in synovial fluid depends, among other things, upon awareness of preceding intraarticular injections crystalline material such as corticosteroid esters. The morphologic and birefringent properties of corticosteriod crystals vary with the specific preparation: Steroid crystals occasionally produce transient clinical synovitis.
Kahn CB, Hollander JL, Schumacher HR. Corticosteroid Crystals in Synovial Fluid. JAMA. 1970;211(5):807–809. doi:10.1001/jama.1970.03170050041009