Inhibition of calcium oxalate crystallization by urinary macromolecules

The crystallization of calcium oxalate (CaOx) was determined in dialyzed urine samples collected between 0600 and 1000 hours from 18 normal men, 10 normal women and 13 men and 10 women with CaOx stone disease. Each urine samples was supersaturated by the addition of calcium chloride and sodium oxalate, and CaOx crystallization was followed by quantification of the [14C]-oxalate remaining in solution for 30 min after supersaturation of the sample. The rate of crystallization was compared with that in physiological saline. The surface area delimited by the urine and saline curves was used to express the inhibition of CaOx crystallization by urinary macromolecules (IUMM). The IUMM was significantly higher in urine from normal women than in that from stone-forming women (P less than 0.05), normal men (P less than 0.005), and stone-forming men (P less than 0.02). However, there were no significant differences between stone-forming men and stone-forming women, nor was IUMM higher in normal men than in stone-forming men. A high concentration of inhibitors might protect women from CaOx stone formation and be one factor explaining the lower stone-formation rate in women. Although low values were more predominate in normal men than in normal women, there were no significant differences between the groups when the inhibition was corrected for differences in urinary volumes.