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The addition of different amounts of hydroxyapatite crystals (HAP) to a solution, metastably supersaturated with respect to calcium oxalate (CaOx) resulted in heterogenous crystallization at seed concentrations exceeding 0.2 mmol/l. The induction period varied between 1 and more than 8 h with the shortest period for a seed concentration of 2 mmol/l. Addition to the system of 1 and 2% of whole urine and citrate in concentrations corresponding to approximately 1% of that found in normal urine inhibited the crystallization for as long as 4 h. In a system supersaturated with respect to calcium phosphate (CaP) the total number of crystals was markedly reduced by citrate concentrations exceeding 0.5 mmol/l. The fractions of medium sized and large crystals were sharply reduced and small crystals predominated at higher citrate concentrations. This might indicate effects of citrate on both crystal growth and crystal aggregation. We conclude that increased citrate concentrations during treatment with alkali leads to a significant inhibition of CaOx growth on HAP as well as to a prevention of the formation of large CaP crystals from solutions supersaturated with respect to CaP.

Urine collected during a 24-h period between 06.00 and 10.00 h from 25 patients with recurrent CaOx stone disease was analysed with respect to calcium, oxalate, magnesium, citrate and creatinine. Urinary excretion of oxalate in relation to creatinine was slightly higher in 24-h urine but the correlation between 24-h and 4-h values was good. Good correlations were also recorded for calcium and citrate, whereas a more variable result was obtained for magnesium. In terms of the risk of forming a supersaturated urine (CaOx risk index), a good correlation was observed between 24-h and 4-h urine samples, although the highest values were found in 24-h urine. As a result of a low mean urine flow between 06.00 and 10.00 h, the highest supersaturation in terms of the AP (CaOx) index was observed in these samples. When the risk of calcium oxalate crystallisation (CaOx-CR) was determined by means of the increment in oxalate concentration required for precipitation of CaOx, 7 of 11 samples had the highest values in the 4-h urine. Samples collected during a 4-h period might thus be useful in the evaluation and follow-up of CaOx stone formers and further studies will show to what extent they can replace 24-h urine collections.

Prophylactic treatment with alkaline citrate in patients with recurrent calcium oxalate (CaOx) stone disease results in reduced CaOx supersaturation and increased urinary citrate. The effects of a single evening dose were compared with those of two and three daily doses in six recurrent CaOx stone formers with hypercalciuria, hypocitraturia or raised calcium/citrate quotients. While on a standardized hospital diet the patients were given 7.5 g (28 mmol) of sodium potassium citrate (URALYT-U) in one, two, and three doses. Fractional urine collections during 24 hours were analyzed for pH, composition, and crystallization risk (CR). All dosage regimens had favourable effects on urinary calcium, citrate, calcium/citrate quotients, and CaOx-CR. The most sustained effect was recorded with three divided doses. Single evening doses resulted in the most pronounced effects between 22.00-06.00 h, thereby counteracting the increased risk of CaOx crystallization during that period. In terms of 24h urine composition the best effect was recorded with alkaline citrate administered three times daily, but because of the favourable response by a single evening dose between 22.00-06.00 h the assumption was made that this dosage regimen might be sufficient to reduce the risk of CaOx crystallization and stone formation. However, the validity of such an assumption can only be established by long-term clinical studies.