Measurement of the risk of calcium oxalate crystallization in urine

The risk of calcium crystallization (CaOx-CR) in urine was analyzed by means of crystal counting following standardized addition of oxalate. CaOx-CR was determined in 24 h urine samples from 21 stone formers and 26 normal subjects following dilution of urine to a creatinine concentration of 5 mumol per ml. The mean (+/- SD) CaOx-CR was in stone formers 1.42 +/- 0.57 and in normal subjects 1.29 +/- 0.40. CaOx-CR was also analyzed in 16 fresh urine samples diluted to 80 per cent of the original concentration whereby values between 0.36 and 3.6 were recorded. There was a good correlation between CaOx-CR and estimates of the ion-activity product of CaOx, both in urine diluted to 5 mumol of creatinine per ml and in 80 per cent diluted urine. It ist suggested that the method described is of value for evaluation and follow up of patients with CaOx urolithiasis.