Differences in 24-hour urine composition between apatite and brushite stone formers

Objective: To determine the differences in 24-hour urine composition between apatite and brushite stone formers. Methods: We conducted a retrospective review of 110 calcium phosphate (CaP) stone formers with both stone composition analysis and 24-hour urinalysis available. Analysis of 24-hour urine constituents between apatite and brushite stone formers was performed using univariable t test and multivariable linear regression models, adjusting for clinical and demographic factors. Results: A total of 97 patients (88%) had predominantly apatite stones and 13 patients (12%) had predominantly brushite stones. In univariable analysis, brushite stone formers had significantly higher mean 24-hour urinary calcium excretion (apatite = 204.8 ± 103.5 mg vs brushite = 329.7 ± 136.6 mg, P =.007), higher mean super saturation (SS) CaP (apatite = 1.423 ± 0.867 vs brushite = 2.576 ± 0.171, P =.004) and lower mean SS uric acid (apatite = 0.688 ± 0.796 vs brushite = 0.345 ± 0.190, P <.001). Similarly in multivariable analysis, brushite stone formers had significantly higher mean 24-hour urinary calcium excretion (mean difference = 135.1 mg, P <.001) and higher mean SS CaP (mean difference = 1.14, P <.001) but similar mean SS uric acid (mean difference = -0.37, P =.103). All other factors analyzed including body-mass index, urinary pH, volume, oxalate, citrate, sodium, potassium, magnesium, phosphate, chloride, ammonium, sulfate, uric acid, and SS calcium oxalate were similar between both stone groups. Conclusion: In a cohort of CaP stone formers, brushite stones were associated with higher urinary calcium excretion and higher urinary SS CaP when compared to apatite stones. Aggressive measures to reduce urinary calcium may be particularly helpful to prevent brushite stone formation.