Prediction by computer modelling of the precipitation of stone-forming solids from urine

Abstract

The development and verification of an equilibrium speciation computer model of urine is described. The model is based on critically selected equilibrium constants and solubility products corrected where needed to T=37°C and I=0.2 mol dm −3. The model represents a slight improvement over previous equilibrium models in that a simpler and more reliable measure is used to represent the degree of supersaturation with respect to a stone-forming salt. A further improvement arises from an implicit allowance being made for certain kinetic factors governing the precipitation of calcium-phosphate solids. The model was verified by comparison with an analogous experimental procedure applied to a standard reference artificial urine. Good predictions were obtained for the precipitation of calcium oxalate monohydrate (COM), dicalcium phosphate dihydrate (DCPD), calcium hydroxyapatite (HAP) and uric acid, but not for struvite. The model indicates that normal urine in the physiological pH range of 5.5 to 7.0 contains precipitates of COM, DCPD and HAP. Precipitates of uric acid and struvite are predicted to occur below and above a pH of 6, respectively. Further computations yield results which are consistent with the view that DCPD may act as a precursor in the precipitation of HAP. Finally, the results indicate that COM precipitation is far more markedly augmented by an increase in oxalate concentration than by an increase in calcium concentration.