Estimation of the state of saturation of brushite and calcium oxalate in urine: a comparison of three methods.

The estimation of the degree of saturation of brushite and of calcium oxalate in urine of patients with disorders of calcium metabolism has been proved to be an effective tool in evaluating the patient's propensity to form renal stones. The methods used in three different laboratories have been compared and evaluated in 27 urine samples. Relative saturation ratio (ratio of activity product and thermodynamic solubility product of brushite) and activity product ratio (ratio of activity product of original sample and that obtained after incubation of sample with synthetic brushite) were determined. Similar studies were performed for calcium oxalate. The values for relative saturation ratio of brushite or calcium oxalate differed widely among the three techniques. However, nearly identical values for activity product ratio of brushite were obtained. The activity product ratios of calcium oxalate, obtained by different techniques, closely approximated each other except in sampels containing excessive amounts of calcium (greater than 5 mM) or oxalate (greater than 5 mM). The values for the relative saturation ratios for both brushite and calcium oxalate were usually higher than the corresponding values for activity product ratio. The results indicate that the use of activity product ratio provides a more simple and reliable estimate of the urinary state of saturation than does the relative saturation ratio.