Repeated urine analysis in patients with calcium stone disease

Objective: To study the variability in urine composition with respect to factors of importance for the calcium salt crystallization process and to test the reliability of using one or several urine samples in the clinical evaluation. Patients and Methods: Twelve patients collected 16-hour daytime and 8-hour night urine samples during 4 days of the same week. The urine was analysed for calcium, oxalate, phosphate, magnesium, citrate and pH, and the ion activity products of CaOx [AP(CaOx) index] and CaP were calculated. The risk of CaOx crystallization, as well as the inhibition of CaOx crystal growth and aggregation, were assessed. Results: There was a good correlation between estimates of the AP(CaOx) index in the different samples, as well as between the AP(CaOx) index and the direct assessment of the risk of CaOx crystallization in the night and daytime urine samples. There was, however, a pronounced intra-individual variation of all variables and parameters. With the assumption that an abnormality would appear in at least one of the four samples, we found that in more than 80% of the cases, two 24-hour (16 + 8 h) urine samples were sufficient to establish whether the patient had a normal or an abnormal urine composition. Conclusion: Urine samples collected during two 24-, 16- or 8-hour periods appear to be useful for detecting biochemical abnormalities considered of importance for CaOx stone formation.