Urine is a saturated equilibrium and not a metastable supersaturated solution: evidence from crystalluria and the general composition of calcium saltand uric acid calculi

A computer algorithm is described which allows urine to be modelled as a sa turated equilibrium solution with respect to any combination of the solids calcium oxalate, calcium hydrogen phosphate (brushite), amorphous calcium p hosphate, uric acid, sodium hydrogen urate and ammonium hydrogen urate. It is demonstrated that this model of urine, unlike the widely accepted metast able supersaturated solution model, explains the long-known calcium salt cr ystalluria versus pH curves of both non-stone-forming and stone-forming uri ne. Further, the saturation model accounts for why most "infection" stones do not contain calcium oxalate and why most "urate" stones are composed sol ely of uric acid and not admired with alkali metal hydrogen urate salts. Th e supersaturation model of urine cannot explain satisfactorily these well-k nown phenomena. For example, the supersaturation model predicts that virtua lly ail "infection" stones should contain calcium oxalate along with calciu m phosphate and, perhaps, struvite.