Studies on citrate metabolism in normal subjects and kidney stone patients

The study was carried out in order to investigate citrate metabolism (i.e. serum levels, filtered load, tubular reabsorption, total and fasting urinary excretion) in a large group of normal subjects (n = 39) and kidney stone patients (n = 45). We found that mean serum citrate levels and filtered loads were similar in the two groups studied. On the other hand, both the 24-hour urinary citrate and the fasting citrate/creatinine ratio were significantly reduced, whereas the mean tubular reabsorption of citrate appeared to be significantly increased in kidney stone patients in respect to control subjects. A good correlation was found between the fasting and the 24-hour citrate/creatinine ratio in both normal subjects (r = 0.84, p less than 0.001) and kidney stone patients (r = 0.89, p less than 0.001). Our data indicate that the kidney appears to be involved in the pathogenesis of hypocitraturia in kidney stone patients and that a substantial proportion of these patients show a reduced urinary excretion of citrate. Finally, the evaluation of the fasting citrate/creatinine ratio may replace the measurement of the substance on the basis of a 24-hour urine collection.