The effect of serum on the crystallization of calcium oxalate in whole human urine: inhibition disguised as apparent promotion.

The effect of serum on calcium oxalate crystallization was studied in whole human urine. At concentrations between 0.005% to 0.10% (v/v), serum had no effect on the metastable limit of urine as detected by the Coulter Counter. However, serum caused a marked increase in the number and volume of calcium oxalate crystals deposited in response to an oxalate load. That this increase in volume reflected true deposition of calcium oxalate was confirmed by determining calcium concentration and by including control urines containing added serum but no exogenous oxalate. Analysis of particle size distributions showed that the crystals deposited in the presence of serum were smaller than those occurring in the control. Since serum did not affect the limit of metastability, the enhancement of crystal volume is unlikely to be a result of heterogeneous nucleation. We hypothesize that the apparent promotion effect of serum is due to its inhibition of crystal aggregation which increases the crystal surface area available for calcium oxalate deposition.