Elucidation of factors determining formation of calcium phosphate stones

In this study we compared the effect of the urinary concentration of calcium with that of oxalate on the urinary relative saturation ratio (RSR) of calcium oxalate and brushite in patients with pure or mixed calcium phosphate stones.

A retrospective data analysis was performed of urinary stone risk analysis in 133 patients with pure or mixed calcium phosphate stones (30% or greater). Correlation was sought between stone events and urinary stone risk factors, and between brushite and urinary constituent RSRs before and after adjustment for variables.

The formation rates of total events and stone surgeries weakly but significantly correlated with RSR urinary brushite (total events r = 0.22, p = 0.01) but not with RSR calcium oxalate. Urinary RSR brushite significantly correlated positively with urinary calcium (r = 0.67, p <0.0001), phosphorus and pH. After adjustment RSR brushite inversely correlated with citrate (r = -0.3, p = 0.0006). However, it did not correlate with oxalate.

In stone formers with calcium phosphate or mixed calcium oxalate-calcium phosphate stones the stone formation rate depends on the urinary saturation of brushite but not of calcium oxalate. RSR brushite is determined by the urinary concentration of calcium, phosphate, citrate and pH but not by oxalate. Hypercalciuria should be controlled in patients with idiopathic calcareous renal calculi who form stones of pure or mixed calcium phosphate.