Critical Reappraisal of Methods for Measuring Urine Saturation with Calcium Salts

Background: Metabolic and physicochemical evaluation is recommended to manage the condition of patients with nephrolithiasis. The estimation of the saturation state (β values) is often included in the diagnostic work-up, and it is preferably performed through calculations. The free concentrations of constituent ions are estimated by considering the main ionic soluble complexes. It is contended that this approach is liable to an overestimation of β values because some complexes may be overlooked. A recent report found that β values could be significantly lowered upon the addition of new and so far neglected complexes, Ca(PO₄)Cit]⁴⁻ and Ca₂H₂(PO₄)₂]. The aim of this work was to assess whether these complexes can be relevant to explaining the chemistry of urine. Methods: The Ca–phosphate–citrate aqueous system was investigated by potentiometric titrations. The stability constants of the parent binary complexes Cacit]⁻ and CaPO₄]⁻, and the coordination tendency of PO₄³⁻ toward Ca(cit)]⁻ to form the ternary complex, were estimated. β_CaOx and β_CaHPO4 were then calculated on 5 natural urines by chemical models, including or not including the CaPO₄]⁻ and Ca(PO₄)cit]⁴⁻ species. Results: Species distribution diagrams show that the Ca(PO₄)cit]⁴⁻ species was only noticeable at pH > 8.5 and below 10% of the total calcium. β values estimated on natural urine were slightly lowered by the formation of CaPO₄]⁻ species, whereas Ca(PO₄)cit]⁴⁻ results were irrelevant. Conclusions: While CaPO₄]⁻ species have an impact on saturation levels at higher pHs, the existence of ternary complex and of the dimer is rejected.