Examination of crystalluria in freshly voided urines of recurrent calcium stone formers and normal individuals using a new filter technique

Purpose: We evaluated calcium oxalate (CaOx) and calcium phosphate (CaP) crystalluria in freshly voided urines of normal individuals (controls) and recurrent calcium stone formers (RSF) using a new filter technique. Materials and Methods: Chemical analysis of urinary sediment retained by a 0.45 mu m. Millipore filter was used to quantitate crystalluria. A CaOx and CaP crystal suspension initially confirmed the reliability of the filter technique. Freshly voided urine samples from 11 controls and 15 RSF were then evaluated. Sediment calcium (S-Ca), oxalate (S-Ox) and phosphate (S-PO4) were compared with routine light microscopy for crystal detection. Results: The recoveries of calcium (Ca) and oxalate (Ox) from the CaOx crystal suspension were 83.8% and 87.5%, respectively. The recoveries of Ca and phosphate (PO,) from the CaP crystal suspension were 97.7% and 89.7%, respectively. The CaOx and CaP crystal volumes (S-Ox and S-PO4) were similar between controls and RSF. However, the sediment to urinary ionic ratios, a parameter indicative of crystal formation under similar levels of supersaturation, was significantly higher in RSF. S-PO4 was 3 to 4 times higher than S-Ox, both in controls and RSF. There was a strong positive correlation between urinary Ox (U-Ox) concentration, and S-Ox and S-Ca concentrations in RSF, but not in controls. There was also a positive correlation between urinary phosphate (U-PO4) and S-PO4 in both groups. A good correlation was also found between the filter technique and standard microscopy for the detection of crystalluria. Conclusions: We conclude that the filter technique is a simple and sensitive quantitative method to study crystalluria. The predominant crystal type in fresh urines of both controls and RSF appears to be calcium phosphate. The principal difference between crystalluria of RSF and normals is its tendency to form at a lower urinary ionic concentration in RSF, suggesting a higher crystallization inhibitor activity in normal individuals.