Ultrafiltration evidence of ion binding by macromolecules in urine.

In an effort to rationalize the difference between computed estimates of urinary calcium oxalate supersaturation and estimates obtained by equilibrating urine with solid calcium oxalate, we examined ultrasfiltered urine for the binding of urinary ions. Urine was passed through a filter with a nominal passage cutoff of 1000 Daltons. The average reduction in concentration of the ultrafiltrate was 12.7 per cent for oxalate, 11.9 per cent for phosphate, 9.4 percent for calcium, 7.6 per cent for magnesium, and 6.9 per cent for sodium. From these results we conclude that calculations of urinary ion equilibrium will give better estimates if the composition of ultrafiltered urine is used.