[Diagnosis of relative hypomagnesurea in calcium lithiasis: reference values for a new parameter]

In calcium stone formers, the ratio of urinary magnesium concentration to urinary calcium concentration (Mg/Ca) is used to express the already reported low Mg excretion relatively to Ca excretion. However the Mg/Ca value depends on the Ca excretion rate, decreases after oral Ca load and is not directly related to Mg excretion. Therefore when studying patients with different Ca excretion rates, or when performing an oral Ca load, Mg/Ca is not directly useful to express a relative hypomagnesuria. We established a new parameter from the normal relationship between urinary Mg and Ca excretion rates. Its value is above - .065 in 97.5% of 99 control urinary samples. It is stable for any studied Ca excretion rate, including post Ca load excretion rates, and is highly correlated with Mg excretion rate. This parameter allows for the accurate diagnosis of relative hypomagnesuria in stone formers.