Toren PJ and Norman RW (2005) Is 24-hour urinary calcium a surrogate marker for dietary calcium intake? Urology 65: 459–462

The negative correlation between dietary calcium intake and risk of recurrent calcium-based stones formed the basis of a recent study that investigated whether 24 h urinary excretion of calcium could form a reliable marker for dietary intake of calcium.

Toren and Norman randomly selected 68 women from their stone clinic. All had attended the clinic at least twice and had been recruited to complete a four-day record of their diet along with two 24 h urine collections on days three and four. The researchers analyzed the dietary data and composition of urine samples provided by participants, and used multiple regression analysis to investigate the relationship between individual dietary nutrients and the mean 24 h urinary calcium level.

A low 24 h urinary calcium level did not always correlate with a low dietary intake of calcium. Furthermore, statistical analysis suggested that the positive predictive value of a patient's 24 h urine calcium level was not sufficient to be considered a reliable surrogate marker for their respective dietary calcium intake.

This study reinforces the need to obtain detailed dietary records for patients with stone formation so that their calcium intake can be modified where necessary, to minimize the risk of stone recurrence.