New method for discriminating between calcium stone formers and healthy individuals

Differences in the free Ca2+ in undiluted urine from stone formers and normal subjects using a new generation of ion-selective electrodes

OBJECTIVES

To measure the ionized Ca2+ in urine from normal subjects and patients with urinary tract stones.

PATIENTS AND METHODS

Urine samples were obtained from 37 normal subjects and 52 stone-formers. Ca2+ was measured using plastic dip-cast ion-selective electrodes; total Ca and other variables were measured by standard analytical techniques.

RESULTS

The ionized Ca2+ and the total Ca were greater in stone-formers' urine although the difference was more significant with the Ca2+ data, especially at a standardized pH. Absolute values of the Ca2+ were dependent on the type of reference electrode used. The Ca2+ was a function of urine pH; lower values were recorded as pH increased. Measured values of the Ca2+ did not correlate well with calculated values using a standard computer program.

CONCLUSIONS

Measurement of the Ca2+ in spot urine samples at a standard pH provides a more discriminative test than total Ca for the presence of urinary tract stones. The pH dependence of the Ca2+ may have important consequences in the formation of urinary tract stones. The poor correlation between measured and computed values of Ca2+ emphasizes the importance of accurate measurement of this variable.