[A study of the risk factors in calcium oxalate stone formation--simple method for measuring metastable limits by the...

The stone forming urinary environment may be conducive to spontaneous nucleation of calcium oxalate, since it is generally characterized by a reduced metastable limit. This study indicates that the microplate method offers simple and reliable measures for estimating calcium oxalate's propensity for spontaneous nucleation. In measuring the metastable limit by the microplate method, 200 microliters aliquots of each urine sample were treated with sodium oxalate to give final concentrations of 0-1.5 mmol. The urine samples were then incubated at 37 degrees C for 20 min and the minimum amount of oxalate necessary to induce nucleation detectable by inverted microscopy was taken to be the measured metastable limit of each urine. The metastable limit by microplate method positively was correlated (p less than 0.001) with the metastable limit found by the Coulter counter method and inversely correlated (p less than 0.001) with the concentration product ratio. The metastable limits were significantly lower (p less than 0.001) in recurrent stone formers than in the control subjects. The metastable limit rose significantly (p less than 0.01) during treatment with thiazides. Moreover, the metastable limit was inversely correlated (p less than 0.01) with the stone episode rate. It is concluded that the metastable limits by the microplate method are useful in stone forming potential identification in urine as well as in the assessment of the response to the therapy.