Is 24-hour urinary calcium a surrogate marker for dietary calcium intake?

Objectives: To determine whether the 24-hour urinary excretion of calcium was a reliable surrogate marker for dietary calcium intake. Although dietary calcium intake has been negatively correlated with the risk of recurrent calcium-based stones, detailed dietary histories are not routinely evaluated in most patients with recurrent stone formation.

Methods: The dietary records and corresponding 24-hour urine collections of 68 randomly selected women with a history of calcium-based renal stones and two or more outpatient clinic visits were studied. Subjects were excluded if they had conditions or took medications affecting calcium absorption or excretion. Multivariate regression analysis was performed on the most recent set of data per patient with 24-hour urinary calcium as the dependent variable. Independent variables included age, weight, and dietary calcium, sodium, potassium, magnesium, fiber, and animal protein. Regression analysis was performed on the differences between the first and last visits for dietary and urinary data. Using tertiles, the positive predictive value of 24-hour urinary calcium for the respective dietary intake tertiles was calculated.

Results: The regression model on absolute values showed all dietary parameters to have squared partial correlation coefficients of less than 0.3 (P = 0.015, R2 = 0.264). In the second regression analysis, the model did not significantly explain the variance (P = 0.656). The positive predictive value of a mean 24-hour urinary calcium level less than 3.75 mmol/L for calcium intake less than 585 mg/day was 45%.

Conclusions: The results of the present study revealed that the 24-hour urinary calcium cannot be used as a surrogate marker for dietary intake of calcium. A detailed dietary history is needed for all women with recurrent stone formation for proper assessment and potential modification of calcium intake to reduce recurrences.