A quantitative study of the phagocytosis of urate crystals in the synovial fluid of asymptomatic joints of patients with gout.

The objective of this study was to determine whether monosodium urate (MSU) crystals are phagocytosed in the synovial fluid (SF) of the asymptomatic joints of patients with gout. SF samples were obtained from 20 asymptomatic knees of 19 different patients. Cell and differential counts were done. Intracellular MSU crystals were identified by ordinary and polarizing light microscopy. We found that in 19 out of the 20 SF samples intracellular MSU crystals have been found. A mean of 22.55% [confidence interval (CI) 13.92, 31.18; range 0-62] of all the cells contained intracellular MSU crystals. The majority of the cells which contained intracellular crystals were mononuclear cells (MC), and polymorphonuclear (PMN) leucocytes containing intracellular crystals accounted only for 0.5% (CI 0, 1.05; range 0-5) of the total. The total cell count was 527 cells/mm3 (CI 226, 828, range;: 30-2670). Poor correlation was found between the percentage of cells with intracellular crystals and both the total cell count (r = -0.22) and the percentage of PMN leucocytes (r = -0.26). We conclude that cells containing phagocytosed MSU crystals--generally mononuclear cells--are a regular finding in the SF of asymptomatic joints of patients with gout. This finding indicates that other factors besides intra-articular interaction between crystals and cells are necessary to produce arthritis in gouty patients.