MEASUREMENT OF METASTABILITY AND GROWTH OF CALCIUM OXALATE IN NATIVE URINE BY CALCIUM ELECTRODE: A NEW APPROACH FOR CLINICAL AND EXPERIMENTAL STONE RESEARCH

Crystallization tests are most important for stone research. Since crystallization properties change with urinary dilution, centrifugation and filtration, the test should be performed in freshly voided and non-pretreated urine. Recently, we have developed an automated method to study the growth of calcium oxalate monohydrate in native urine. However, because seed crystals always become coated with urinary macromolecules, the study of crystals directly produced in urine may be more relevant. In a new modification of our test system, calcium oxalate crystallization was induced in freshly voided unseeded urine by the addition of varying exogenous oxalate loads (Ox ex ) and the decrease of ionic calcium (∆ Ca 2+ ) was automatically monitored during crystallization. Calcium chelation was determined by measuring Ca 2+ during calcium titration (Ca ex ). With oxalate loads inducing crystallization without any detectable induction time, straight linear regressions of t/ ∆ Ca t 2+ vs t were observed, allowing the calculation of ∆ Ca 2+ at infinite observation time (∆ Ca∞ 2+ ) and its half time (h). Further linear regressions were found with respect of Ca 2+ vs Ca ex , ∆ Ca∞ 2+ vs Ox ex and 1/h vs ∆ Ca∞ 2+ . These regressions allowed calculation of a chelation coefficient (c), a limit of urinary metastability (m) and a growth rate constant (g) as well as the description of oxalate induced crystallization by a simple equation. This new approach has been compared to other methods studying nucleation and growth of calcium oxalate in urine.