Hydrochlorothiazide therapy in nephrolithiasis effect on the urinary activity product and formation product of brushite

The effect of hydrochlorothiazide on the formation of renal stones containing calcium was evaluated by the quantitative assessment of the propensity of urine for the nucleation of brushite (nidus of calcium stones). In 6 patients with renal stones, the urinary activity product ratio (APR) and the formation product ratio (FPR) of brushite were measured before, during, and following treatment with hydrochlorothiazide (50 mg twice a day orally) for from 5 days to 1 month. The urinary APR (the state of saturation with respect to brushite) decreased in the majority of cases primarily as the result of a fall in urinary calcium. The urinary FPR (lowest state of supersaturation at which nucleation is initiated) usually increased, probably as a result of a rise in urinary pyrophosphate. Both changes reduced the propensity for the formation of the nidus of brushite, and may therefore account for the improvement reported after thiazide therapy in nephrolithiasis.