Spontaneous precipitation of brushite in urine: evidence that brushite is the nidus of renal stones originating as calcium phosphate.

Further evidence that brushite plays a regulatory role in renal stone formation was provided by the identification of brushite as the first precipitate that appears in supersaturated urine by spontaneous precipitation. Calcium chloride was added to induce supersaturation in urine specimens from twelve subjects with and twelve subjects without nephrolithiasis. The first precipitate in all specimens with pH below 6.9 was identified as brushite by x-ray diffraction and shown to have a calcium-phosphorus ratio of approximately 1.0. The activity product of [Ca(2+)] x [HPO(4) (2-)] necessary to produce a precipitate ranged from 2.2 to 3.5 times the solubility product of brushite, but the range and mean were the same for both groups of subjects. The activity product of [Ca(2+)] x [HPO(4) (2-)] in the supernatant (after spontaneous precipitation) was not significantly different from that obtained after incubation of the same urine specimen with synthetic brushite. These results provide conclusive evidence that brushite constitutes the solid phase formed by spontaneous precipitation from acidic urine supersaturated with respect to calcium and phosphorus; they suggest that the nidus for calcium-containing renal stones is brushite as well.