Evaluation of factors involved in calcium stone formation.

The role of urinary calcium oxalate supersaturation in the initiation of calcium stone formation is unclear since 24-hour urine collections from both stone-formers and nonstone-formers are supersaturated for calcium oxalate. Urinary supersaturation for calcium phosphate, uric acid, and monosodium urate or decreased excretion of inhibitors of stone formation, citrate and magnesium, may also contribute to calcium stone formation. To assess these possibilities we studied variations within 24 h of determinants for calcium stone formation. Six consecutive 4-hour urine specimens were collected from stone-formers (n = 15) and nonstone-formers (n = 11) during their usual diet and activities. Compared to nonstone-formers, urine from stone-formers had increased calcium concentrations, while pH and oxalate, phosphorus, uric acid, magnesium and citrate concentrations were not different. Urine from both stone-formers and nonstone-formers was always supersaturated for calcium oxalate, uric acid and monosodium urate with no difference between the groups. Urine from stone-formers was always supersaturated for calcium phosphate as brushite, while urine from nonstone-formers was mostly undersaturated for brushite. In addition to these physicochemical studies, we reviewed the composition of 235 stones containing calcium oxalate and analyzed it by crystallography. The center of 67% of these stones contained calcium phosphate. These physicochemical and stone analysis data suggest that precipitation of a calcium phosphate crystal nidus may contribute to the initiation of many calcium stones.