The composition of four-hour urine samples from patients with calcium oxalate stone disease.

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.