In nine hypercalciuric patients with renal stone disease non-fasting serum and urine calcium were significantly raised. After overnight fasting, both serum and urine calcium values had fallen almost into the normal range and calcium deprivation for a further eight hours yielded completely normal values in all but one case. Oral administration of a large dose of calcium citrate (100 mg. calcium/kg.) produced a significant rise in serum and urine calcium in both groups, but the rise was significantly greater in the hypercalciuric subjects than in the controls. The cumulative eight-hour calcium excretion after the oral load was more than 174 mg. in all the hypercalciuric cases and less than this amount in the controls. These observations tend to show that idiopathic hypercalciuria is absorptive in origin and emphasize the importance of measuring serum and urine calcium in the fasting state. It is suggested that oral calcium loading may constitute a better diagnostic test of this condition than a 24-hour urine collection on a free diet.
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