Propensity for spontaneous nucleation of calcium oxalate: Quantitative assessment by urinary FPR-APR discriminant score

The stone-forming urinary environment may be conducive to spontaneous nucleation of calcium salts, since it is generally characterized by a reduced limit of metastability (FPR or minimum supersaturation required for initiation of spontaneous nucleation) and an increased saturation (APR) of calcium oxalate and brushite (CaHPO₄·2H₂O). A discriminant analysis of the relationship between FPR and APR yielded a FPR-APR discriminant score, which gave a quantitative measure of the propensity for spontaneous nucleation. Positive scores, indicative of the “likelihood” of spontaneous nucleation, were found in the majority of patients with hypercalciuria in whom stones formed. Negative scores, representative of the “unlikelihood” of nucleation, were found in the majority of control subjects. Treatment with thiazide or sodium cellulose phosphate significantly reduced the discriminant score for both calcium oxalate and brushite, a finding which suggested that the propensity for spontaneous nucleation of calcium salts was retarded. Allopurinol lowered the score for calcium oxalate but not for brushite. Moreover, the FPR-APR discriminant score for calcium oxalate was positively correlated with the stone formation rate. It is concluded that FPRAPR discriminant scores are useful in the identification of stone-forming potential in urine, and in the assessment of response to therapy.