Effects of urine dilution on quantity, size and aggregation of calcium oxalatecrystals induced in vitro by an oxalate load.

Increasing urinary volume is an important tool in the prevention of calcium renal stones. However, the mechanism of how it actually works ís only partially understood.This study aimed at assessing how urine dilution affects urinary calcium oxalate crystallization. A total of 16 male idiopathic calcium oxalate (CaOx) stone-formers and 12 normal male subjects were studied and 4 h urine samples were taken twice. under low (undiluted urine) and high hydration conditions (diluted urine). An equal oxalate load (1.3 mmol/L) was added to both types of urine and the crystallization parameters were assessed. In both stone-formers and normal subjects, the crystallization processes were significantly (p<0.05 or less) more marked in the undiluted urine than in the diluted urine ín terms of: a) total quantity of calcium oxalate dihydrate (COD) and calcíum oxaf ate monohydrate (COM) crystals; b) total quantity of crystalline aggregares; and c) aggregation index (i.e., ratio betwee-nihe area occupied by crystalline aggregates and the area occupied by all the crystals present). The comparison between stone-formers and normal subjects showed that the greatest difference was for the size of COD crystals, whích were larger in the urine of the stoneformers. A further important finding was an inverse relationship between changes in urinary volume and in the aggregation index (r:-0.53, p:0.004). In conclusion. urine dilution considerably reduces crystallization phenomena induced in vitro by an oxalate load in both calcium stone-formers and normal subjects.