Comparison of Semi-Empirical and Computer Derived Methods for Estimating Urinary Saturation of Calcium Oxalate

Purpose: Estimating calcium oxalate saturation in human urine is critical for nephrolithiasis clinical research and practice. The Joint Expert Speciation System (Mayhem Unit Trust and Council for Scientific and Industrial Research, Pretoria, South Africa) computer program has questioned the validity of the widely used EQUIL 2 program in estimating calcium oxalate urinary saturation. To attempt resolution the computer model based supersaturation index (by the Joint Expert Speciation System) and the relative saturation ratio (by EQUIL 2) were compared with the experimentally derived activity product ratio, that is the ratio of activity products before and after incubating urine with synthetic calcium oxalate. Materials and Methods: To determine the experimental conditions required to attain calcium oxalate steady state solubility the filtrate concentration product of calcium and oxalate was determined after incubating 8 urine samples with 2 to 15 mg/ml calcium oxalate for various intervals. In 20 urine samples the activity product ratio of calcium oxalate was compared with the relative saturation ratio and the supersaturation index. Results: Steady state solubility occurred after incubating 15 mg calcium oxalate per ml urine for 72 hours. The mean ± SD supersaturation index of 4.92 ± 2.57 in the original urine samples closely approximated the activity product ratio of 5.08 ± 3.10 but the relative saturation ratio of 7.47 ± 3.89 was significantly higher than the activity product ratio. The supersaturation index was recalculated after omitting soluble complexes recognized by the Joint Expert Speciation System but not by EQUIL 2, including Ca₂H₂(PO₄)₂ and (CaCitPO₄)^4-. The corrected supersaturation index was compared with the relative saturation ratio. After correction the supersaturation index increased to 7.28 ± 3.81, which was not significantly different from the relative saturation ratio. Conclusions: The Joint Expert Speciation System is more accurate than EQUIL 2 to estimate calcium oxalate saturation, probably by accounting for Ca₂H₂(PO₄)₂, (CaCitPO₄)^4-, and other minor complexes of calcium and oxalate.