THE TWO crystal deposition diseases are gout, the prototype disease, and pseudogout (articular chondrocalcinosis) a new entity delineated by Mc-Carty and associates.1 The offending crystals are sodium urate and calcium pyrophosphate, respectively. When either gout or pseudogout presents as acute arthritis involving a joint accessible to aspiration, the physician has an unusual opportunity to establish the diagnosis quickly. Examined by means of polarized light microscopy, a drop of synovial fluid beneath a cover slip rimmed with material to retard evaporation should demonstrate birefringent, intracellular and extracellular crystals.2
Fluid obtained intercritically or from a chronic effusion may not reveal urate or calcium pyrophosphate crystals and, if present, these are likely to be extracellular and few in number.1,3 Confusing structures such as cartilaginous fragments, cholesterol crystals, and small numbers of other birefringent crystals of yet unknown significance are seen occasionally extracellularly.2 However these present no problem in the