Renal Phosphate Control as a Reliable Predictive Factor of Stone Recurrence

Purpose

Recent studies have suggested a defect in phosphate balance as a significant underlying cause of calcium urolithiasis. In this study we assessed the influence of decreased renal phosphate reabsorption capacity on urinary metabolic abnormalities as well as on stone recurrence.

Materials and Methods

A database of patient history, and serum and urine chemistry studies was analyzed for 950 consecutive stone formers and 91 normal controls. The maximal reabsorption of phosphate by the glomerular filtration rate was calculated, and the effect of maximal reabsorption of phosphate by the glomerular filtration rate on stone metabolites and stone recurrence was determined. A value of maximal reabsorption of phosphate by the glomerular filtration rate lower than 1.83 mg/dl was defined as a low rate. Of the stone formers 266 (28.0%) who had been followed for more than 36 months (median 49, range 2 to 152) were included in the recurrence analysis.

Results

Maximal reabsorption of phosphate by the glomerular filtration rate was significantly less in stone formers compared to normal controls, and was negatively correlated with the urinary excretion of uric acid, calcium and phosphate. Of the stone formers 13.1% (124 of 950) had a low maximal reabsorption of phosphate by the glomerular filtration rate as well as a higher prevalence of hypercalciuria and hyperuricosuria than individuals with normal maximal reabsorption of phosphate by the glomerular filtration rate. The multivariate Cox regression model revealed that the low maximal reabsorption of phosphate by the glomerular filtration rate (hazards ratio 1.685, 95% CI 1.040–2.730, p = 0.034) was a strong predictor of stone recurrence in stone formers.

Conclusions

This study demonstrated that the renal phosphate reabsorption capacity was significantly decreased in stone formers compared to normal controls, and that a low maximal reabsorption of phosphate by the glomerular filtration rate was associated with hypercalciuria and hyperuricosuria. Furthermore, renal phosphate handling was an independent predictive determinant for recurrence in stone formers.

Key Words: phosphates, recurrence, urolithiasis

Abbreviations and Acronyms: Npt2a, sodium-dependent phosphate cotransporter 2a, SF, stone formers, TmP/GFR, maximal reabsorption of phosphate by the glomerular filtration rate